drivers: pinctrl_nrf: Configure QSPI CSN pin as output set high

... so that the pin keeps the inactive state of the CSN line when it is
not controlled by the QSPI peripheral and the driver is not suspended.

Currently, the nrf_qspi_nor shim deinitializes the nrfx_qspi driver
after each operation, what leaves the pin uncontrolled until a new
operation is requested (and the nrfx_qspi driver is initialized again)
or the driver is suspended (and the pin is put into low-power state).
This can cause the flash chip to needlessly consume current when there
is no pull-up resistor on its CSN line and the line appears active.
Prevent this by keeping the pin in a defined (inactive) state.

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