esp32c3: add support for this chip

This change adds support for the ESP32-C3, a new chip from Espressif. It
is a RISC-V core so porting was comparatively easy.

Most peripherals are shared with the (original) ESP32 chip, but with
subtle differences. Also, the SVD file I've used gives some
peripherals/registers a different name which makes sharing code harder.
Eventually, when an official SVD file for the ESP32 is released, I
expect that a lot of code can be shared between the two chips.

More information: https://www.espressif.com/en/products/socs/esp32-c3

TODO:
  - stack scheduler
  - interrupts
  - most peripherals (SPI, I2C, PWM, etc)

src/runtime/runtime_esp32c3.go

@@ -0,0 +1,65 @@
+// +build esp32c3
+
+package runtime
+
+import (
+	"device/esp"
+	"device/riscv"
+)
+
+// This is the function called on startup after the flash (IROM/DROM) is
+// initialized and the stack pointer has been set.
+//export main
+func main() {
+	// This initialization configures the following things:
+	// * It disables all watchdog timers. They might be useful at some point in
+	//   the future, but will need integration into the scheduler. For now,
+	//   they're all disabled.
+	// * It sets the CPU frequency to 160MHz, which is the maximum speed allowed
+	//   for this CPU. Lower frequencies might be possible in the future, but
+	//   running fast and sleeping quickly is often also a good strategy to save
+	//   power.
+	// TODO: protect certain memory regions, especially the area below the stack
+	// to protect against stack overflows. See
+	// esp_cpu_configure_region_protection in ESP-IDF.
+
+	// Disable Timer 0 watchdog.
+	esp.TIMG0.WDTCONFIG0.Set(0)
+
+	// Disable RTC watchdog.
+	esp.RTC_CNTL.RTC_WDTWPROTECT.Set(0x50D83AA1)
+	esp.RTC_CNTL.RTC_WDTCONFIG0.Set(0)
+
+	// Disable super watchdog.
+	esp.RTC_CNTL.RTC_SWD_WPROTECT.Set(0x8F1D312A)
+	esp.RTC_CNTL.RTC_SWD_CONF.Set(esp.RTC_CNTL_RTC_SWD_CONF_SWD_DISABLE)
+
+	// Change CPU frequency from 20MHz to 80MHz, by switching from the XTAL to
+	// the PLL clock source (see table "CPU Clock Frequency" in the reference
+	// manual).
+	esp.SYSTEM.SYSCLK_CONF.Set(1 << esp.SYSTEM_SYSCLK_CONF_SOC_CLK_SEL_Pos)
+
+	// Change CPU frequency from 80MHz to 160MHz by setting SYSTEM_CPUPERIOD_SEL
+	// to 1 (see table "CPU Clock Frequency" in the reference manual).
+	// Note: we might not want to set SYSTEM_CPU_WAIT_MODE_FORCE_ON to save
+	// power. It is set here to keep the default on reset.
+	esp.SYSTEM.CPU_PER_CONF.Set(esp.SYSTEM_CPU_PER_CONF_CPU_WAIT_MODE_FORCE_ON | esp.SYSTEM_CPU_PER_CONF_PLL_FREQ_SEL | 1<<esp.SYSTEM_CPU_PER_CONF_CPUPERIOD_SEL_Pos)
+
+	clearbss()
+
+	// Initialize main system timer used for time.Now.
+	initTimer()
+
+	// Initialize the heap, call main.main, etc.
+	run()
+
+	// Fallback: if main ever returns, hang the CPU.
+	abort()
+}
+
+func abort() {
+	// lock up forever
+	for {
+		riscv.Asm("wfi")
+	}
+}