drivers: flash: nrf_qspi_nor: support read of sub-word lengths

mcuboot and possibly other tools read single byte values to determine
the state of objects.  Rather than fail to do the read of values too
short for this peripheral detect the situation and read into a stack
buffer that meets the length criteria, and on success copy the data
into the provided buffer.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>