drivers: mspi_dw: Add waiting for clock in nRF EXMIF specific resume

After the START task is triggered, the clock that drives the SSI core
needs some time to become ready. Before that, writes to SSI registers
may be unsuccessful. Add a loop that performs test writes to one of
the registers after the EXMIF peripheral is resumed to ensure that it
is fully operable.

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