runtime/atsamd51: enable CMCC cache for greatly improved performance on SAMD51

Signed-off-by: deadprogram <ron@hybridgroup.com>

src/runtime/runtime_atsamd51.go

@@ -27,6 +27,7 @@ func init() {
 	initSERCOMClocks()
 	initUSBClock()
 	initADCClock()
+	enableCache()
 
 	cdc.EnableUSBCDC()
 	machine.USBDev.Configure(machine.UARTConfig{})
@@ -367,6 +368,10 @@ func initADCClock() {
 		sam.GCLK_PCHCTRL_CHEN)
 }
 
+func enableCache() {
+	sam.CMCC.CTRL.SetBits(sam.CMCC_CTRL_CEN)
+}
+
 func waitForEvents() {
 	arm.Asm("wfe")
 }