drivers: nrf_qspi_nor: Prevent reading status before sending RDPD

After entering the Deep Power-down mode, some flash chips ignore all
commands except from the one that releases the chip from the DP mode
and it is not possible to successfully read their Status Register then.
Since the QSPI peripheral tries to read this register when it is being
activated, it consequently fails to send the actual command that would
release the flash chip from the DP mode if that is to be done right
after QSPI initialization.
Prevent this problem by performing the QSPI activation with all pins
disconnected. This causes that the Status Register value is read as
all zeros and allows the activation to always finish successfully,
and the RDPD command to be properly sent.

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