flash: nrf_qspi_nor: wait for erase to complete

Wait until the erase operation triggered by `qspi_erase` completes
before returning. This aligns with the behaviour of other flash drivers
like `spi_nor` and `soc_flash_nrf`.

A delay is added to the `qspi_wait_while_writing` to prevent the check
from monopolising the CPU.

Signed-off-by: Jordan Yates <jordan@embeint.com>