drivers: spi_nrfx_*: Add support for optional WAKE line

Add option to use (by defining the `wake-gpios` devicetree properties)
an additional signal line between SPI master and SPI slave that allows
the latter to stay in low-power state and wake up only when a transfer
is to occur.

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