maixbit (uart): working on data tx

When the data to send is too long the program gives an exception.

src/runtime/interrupt/interrupt_k210.go

@@ -0,0 +1,19 @@
+// +build k210
+
+package interrupt
+
+import "device/kendryte"
+
+// Enable enables this interrupt. Right after calling this function, the
+// interrupt may be invoked if it was already pending.
+func (irq Interrupt) Enable() {
+	// TODO: Use current hartid
+	kendryte.PLIC.TARGET_ENABLES[0].ENABLE[irq.num/32].SetBits(1 << (uint(irq.num) % 32))
+}
+
+// SetPriority sets the interrupt priority for this interrupt. A higher priority
+// number means a higher priority (unlike Cortex-M). Priority 0 effectively
+// disables the interrupt.
+func (irq Interrupt) SetPriority(priority uint8) {
+	kendryte.PLIC.PRIORITY[irq.num].Set(uint32(priority))
+}