drivers/flash/nrf_qspi_nor: Add support for nRF53 Series SoCs

Improve the way the nrf_qspi_nor driver configures the SCK frequency,
to properly support QSPI also on nRF53 Series SoCs that use a different
base clock frequency (96 MHz).
Add also a relevant configuration in the spi_flash sample so that it
can run on the nRF5340 DK.

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