drivers: flash: spi_nor: boot into DPD

Boot into the deep power down state when `SPI_NOR_IDLE_IN_DPD` is not
enabled. DPD is the correct hardware state for the `SUSPENDED` software
state. Without this change, it takes a cycle of
`SUSPENDED->ACTIVE->SUSPENDED` to get to the low power state.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>