drivers: flash: spi nor: Add MultInstance support

Modify the SPI Nor driver to be able to have multiple instances at
the same time.

This patch is heavily inspired by the at45 driver.
It was tested on the nRF5340 DK by using the external spi memory two times.
Macros were improved by de-nordic

Signed-off-by: Mehdi Zemzem <mehdi.zemzem2@gmail.com>