nrf5/drivers/ticker: Removing LowPriority callback from nrf51 as there is only one SoftwareIRQ free if bluetooth stack is enabled. Also setting new IRQ priority on SlowTicker to 3 instead of 2, to interleave with bluetooth stack if needed. Updating all NVIC calls to use hal_irq.h defined static inlines instead of direct access.
nrf5/drivers/ticker: Removing LowPriority callback from nrf51 as there is only one SoftwareIRQ free if bluetooth stack is enabled. Also setting new IRQ priority on SlowTicker to 3 instead of 2, to interleave with bluetooth stack if needed. Updating all NVIC calls to use hal_irq.h defined static inlines instead of direct access.

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