drivers: timer: nrf_rtc_timer: Change HAL to HALY in RTC timer driver

NRFX has introduced HALY software layer which is a superset of HAL
and aggregates some hardware registers manipulations in a single
function calls.

Quote from nrfx changelog:

"HALY is an extension of the HAL layer that aggregates basic hardware
use cases within single functions. Now it is used instead of HAL
in the corresponding drivers."

This commit zephyr's driver to be aligned with the approach used
in nrfx, where drivers has been switched to use HALY instead of HAL.

Signed-off-by: Adam Wojasinski <adam.wojasinski@nordicsemi.no>