drivers: timer: nrf_grtc_timer: Align Zephyr to new AUTOEN read manner

The new GRTC reading manner of the SYSCOUNTER uses hardware mechanism which
allows to keep it alive when any of CPUs is not sleeping. Otherwise
the SYSCOUNTER goes into sleep mode. Thus there is no
longer need to maintain the `CONFIG_NRF_GRTC_SLEEP_ALLOWED` symbol, however
if the user wants to have the SYSCOUNTER enabled all the time the
`CONFIG_NRF_GRTC_ALWAYS_ON` can be used instead.
The nrfx_grtc  driver no longer provides the `wakeup-read-sleep` reading
manner.
Also setting the GRTC clock source is performed by the nrfx_grtc driver so
it has been removed from the `sys_clock_driver_init()` function.

Signed-off-by: Adam Kondraciuk <adam.kondraciuk@nordicsemi.no>