drivers: flash: nrf_qspi_nor: Add support for device power management

Deinitialize the nrfx_qspi driver for periods when the device
is suspended. For flash chips with "has-dpd" property set, when
suspending/resuming the device, issue also the enter/exit Deep
Power-down Mode command, respectively.

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