cpu/nrf5x_common: fix uart_poweroff()

Previously, uart_poweroff() and uart_poweron() were no-ops. This
replaces them with the logic to indeed power on and power off the UART
device.

cpu/nrf5x_common/periph/uart.c

@@ -276,6 +276,10 @@ static void _write_buf(uart_t uart, const uint8_t *data, size_t len)
 void uart_write(uart_t uart, const uint8_t *data, size_t len)
 {
     assert(uart < UART_NUMOF);
+    if (!dev(uart)->ENABLE) {
+        /* Device is powered down. Writing anyway would deadlock */
+        return;
+    }
 #ifdef MODULE_PERIPH_UART_NONBLOCKING
     for (size_t i = 0; i < len; i++) {
         /* in IRQ or interrupts disabled */
@@ -337,6 +341,11 @@ void uart_poweron(uart_t uart)
     assert(uart < UART_NUMOF);
 
     if (isr_ctx[uart].rx_cb) {
+#if !defined(CPU_MODEL_NRF52832XXAA) && !defined(CPU_FAM_NRF51)
+        dev(uart)->ENABLE = UARTE_ENABLE_ENABLE_Enabled;
+#else
+        NRF_UART0->ENABLE = UART_ENABLE_ENABLE_Enabled;
+#endif
         dev(uart)->TASKS_STARTRX = 1;
     }
 }
@@ -346,6 +355,11 @@ void uart_poweroff(uart_t uart)
     assert(uart < UART_NUMOF);
 
     dev(uart)->TASKS_STOPRX = 1;
+#if !defined(CPU_MODEL_NRF52832XXAA) && !defined(CPU_FAM_NRF51)
+    dev(uart)->ENABLE = UARTE_ENABLE_ENABLE_Disabled;
+#else
+    NRF_UART0->ENABLE = UART_ENABLE_ENABLE_Disabled;
+#endif
 }
 
 int uart_mode(uart_t uart, uart_data_bits_t data_bits, uart_parity_t parity,
@@ -446,6 +460,7 @@ void uart_poweron(uart_t uart)
 
     if (isr_ctx.rx_cb) {
         NRF_UART0->TASKS_STARTRX = 1;
+        NRF_UART0->POWER = 1;
     }
 }
 
@@ -454,6 +469,7 @@ void uart_poweroff(uart_t uart)
     (void)uart;
 
     NRF_UART0->TASKS_STOPRX = 1;
+    NRF_UART0->POWER = 0;
 }
 
 int uart_mode(uart_t uart, uart_data_bits_t data_bits, uart_parity_t parity,