drivers: flash_mspi_nor: Add support for "supply-gpios" property

Add support for supplying power to the flash chip by activation of
a GPIO specified through the "supply-gpios" property. Implementation
of gpio_reset() is also slightly modified so that it is consistent
with soft_reset() and the new power_supply() and so that all these
functions can use a common routine that performs a reset recovery
delay.

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