drivers: flash: nrf_qspi_nor: Add custom API function to allow XIP

Normally, the driver deactivates the QSPI peripheral for periods when
no QSPI operation is performed. This is done to avoid increased current
consumption when the peripheral is idle. For the same reason, the base
clock on nRF53 Series SoCs (HFCLK192M) is configured for those periods
with the default /4 divider that cannot be used otherwise. However,
when XIP accesses are used, the driver must be prevented from doing
both these things as that would make XIP to fail. Hence, a function
is provided so that applications can inform the driver that XIP is
needed and the above idle actions should be suppressed.
This function (`nrf_qspi_nor_xip_enable()`) replaces the old one
(`nrf_qspi_nor_base_clock_div_force()`) that was intended for similar
purpose but after deactivation of the peripheral was introduced in
commit 95d867e8 it became useless.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>