nRF5x Series: Temporary System Clock driver using BLE Controller Ticker.

[zephyrbot]

Reported by Vinayak Kariappa Chettimada:

nRF5x Series: System Clock driver using BLE Controller Ticker module.

As an OS kernel i want a low power system clock driver using a dedicated RTC on nRFx series so that i can have time awareness while the kernel sleeps during cpu idle.

SysTick on nRF5x Series are absent or off during CPU sleep, hence the kernel should use NRF_TIMER (high current consumption) or NRF_RTCx (low power low precision) as system clock.

BLE Controller implements Ticker that shares one NRF_RTC as multi-user tickless timer. Ticker has a spare independent instance that can be used as system clock. Sharing a NRF_RTC reduces power consumption. As ticker uses "work" abstraction, the second instance is setup to execute at lower ISR priority which alleviates (to acceptable) Controller ISR latencies.

(Imported from Jira ZEP-743)

[zephyrbot]

Related to GH-2222