fe310: increase CPU frequency from 16MHz to 320MHz

This chip can run so much faster! Let's update the default frequency.

Also, change the UART implementation to be more fexible regarding the
clock frequency.

src/machine/machine_fe310.go

@@ -9,7 +9,7 @@ import (
 )
 
 func CPUFrequency() uint32 {
-	return 16000000
+	return 320000000 // 320MHz
 }
 
 const (
@@ -61,9 +61,17 @@ var (
 )
 
 func (uart *UART) Configure(config UARTConfig) {
-	// Assuming a 16Mhz Crystal (which is Y1 on the HiFive1), the divisor for a
-	// 115200 baud rate is 138.
-	sifive.UART0.DIV.Set(138)
+	if config.BaudRate == 0 {
+		config.BaudRate = 115200
+	}
+	// The divisor is:
+	//   fbaud = fin / (div + 1)
+	// Restating to get the divisor:
+	//   div = fin / fbaud - 1
+	// But we're using integers, so we should take care of rounding:
+	//   div = (fin + fbaud/2) / fbaud - 1
+	divisor := (CPUFrequency()+config.BaudRate/2)/config.BaudRate - 1
+	sifive.UART0.DIV.Set(divisor)
 	sifive.UART0.TXCTRL.Set(sifive.UART_TXCTRL_ENABLE)
 	sifive.UART0.RXCTRL.Set(sifive.UART_RXCTRL_ENABLE)
 	sifive.UART0.IE.Set(sifive.UART_IE_RXWM) // enable the receive interrupt (only)