gracefully end() uart, wait for TX & RX phy to power down before disable it

this save around 900uA of power consumption

Author: hathach

cores/nRF5/Uart.cpp

@@ -156,9 +156,15 @@ void Uart::end()
 
   nrfUart->INTENCLR = UARTE_INTENSET_ENDRX_Msk | UARTE_INTENSET_ENDTX_Msk;
 
+  nrfUart->EVENTS_RXTO = 0;
+  nrfUart->EVENTS_TXSTOPPED = 0;
+
   nrfUart->TASKS_STOPRX = 0x1UL;
   nrfUart->TASKS_STOPTX = 0x1UL;
 
+  // Wait for TXSTOPPED event and for RXTO event
+  while ( !(nrfUart->EVENTS_TXSTOPPED && nrfUart->EVENTS_RXTO) ) yield();
+
   nrfUart->ENABLE = UARTE_ENABLE_ENABLE_Disabled;
 
   nrfUart->PSEL.TXD = 0xFFFFFFFF;