Nimble: Compatibility with LFRC calibration

Nimble has to be aware of the low frequency clock
calibration procedure, in order to not interfere with
the usage of the HFCLK. For more info, see
https://github.com/apache/mynewt-nimble/issues/1207

src/libs/mynewt-nimble/nimble/drivers/nrf52/src/ble_phy.c

@@ -37,6 +37,7 @@
 #else
 #include "core_cm4.h"
 #endif
+#include <legacy/nrf_drv_clock.h>
 
 #if MYNEWT_VAL(BLE_LL_CFG_FEAT_LE_CODED_PHY)
 #if !MYNEWT_VAL_CHOICE(MCU_TARGET, nRF52840) && !MYNEWT_VAL_CHOICE(MCU_TARGET, nRF52811)
@@ -2101,7 +2102,7 @@ ble_phy_rfclk_enable(void)
 #if MYNEWT
     nrf52_clock_hfxo_request();
 #else
-    NRF_CLOCK->TASKS_HFCLKSTART = 1;
+    nrf_drv_clock_hfclk_request(NULL);
 #endif
 }
 
@@ -2111,6 +2112,6 @@ ble_phy_rfclk_disable(void)
 #if MYNEWT
     nrf52_clock_hfxo_release();
 #else
-    NRF_CLOCK->TASKS_HFCLKSTOP = 1;
+    nrf_drv_clock_hfclk_release();
 #endif
 }