targets,runtime,machine: add support for the stm32f469-disco board

The LEDs and button work; I haven't tested the SPI and I2C
configuration.

src/runtime/runtime_stm32f469.go

@@ -0,0 +1,28 @@
+//go:build stm32f4 && stm32f469
+// +build stm32f4,stm32f469
+
+package runtime
+
+import "device/stm32"
+
+/*
+   clock settings
+   +-------------+--------+
+   | HSE         | 8mhz   |
+   | SYSCLK      | 180mhz |
+   | HCLK        | 180mhz |
+   | APB2(PCLK2) | 90mhz  |
+   | APB1(PCLK1) | 45mhz  |
+   +-------------+--------+
+*/
+const (
+	HSE_STARTUP_TIMEOUT = 0x0500
+	// PLL Options - See RM0386 Reference Manual pg. 148
+	PLL_M    = 8 // PLL_VCO = (HSE_VALUE or HSI_VALUE / PLL_M) * PLL_N
+	PLL_N    = 360
+	PLL_P    = 2 // SYSCLK = PLL_VCO / PLL_P
+	PLL_Q    = 7 // USB OTS FS, SDIO and RNG Clock = PLL_VCO / PLL_Q
+	PLL_R    = 6 // DSI
+	PLL_CFGR = PLL_M | (PLL_N << stm32.RCC_PLLCFGR_PLLN_Pos) | (((PLL_P >> 1) - 1) << stm32.RCC_PLLCFGR_PLLP_Pos) |
+		(1 << stm32.RCC_PLLCFGR_PLLSRC_Pos) | (PLL_Q << stm32.RCC_PLLCFGR_PLLQ_Pos) | (PLL_R << stm32.RCC_PLLCFGR_PLLR_Pos)
+)