drivers: flash_mspi_nor: Make transfer timeout configurable

Although the value currently hard-coded in the driver (10 ms) is quite
high, it may turn out to be insufficient when there is a need to use
some very low SCK frequency, like 250 kHz.
Make the timeout value configurable per-instance, via devicetree.

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