NRF51822: HAL: Restore WEAK declaration for hal_(deep)sleep

Otherwise we have a conflict between NRF51822 and DELTA_DFCM_NNN40.

targets/TARGET_NORDIC/TARGET_MCU_NRF51822/sleep.c

@@ -18,7 +18,7 @@
 #include "mbed_interface.h"
 #include "toolchain.h"
 
-void hal_sleep(void)
+MBED_WEAK void hal_sleep(void)
 {
     // ensure debug is disconnected if semihost is enabled....
     NRF_POWER->TASKS_LOWPWR = 1;
@@ -26,7 +26,7 @@ void hal_sleep(void)
     __WFE();
 }
 
-void hal_deepsleep(void)
+MBED_WEAK void hal_deepsleep(void)
 {
     hal_sleep();
     //   NRF_POWER->SYSTEMOFF=1;