drivers: spi: context: Add helper for CS GPIO PM

Introduce spi_context_cs_get() and spi_context_cs_put() which shall
be used from drivers to get/put the GPIO port the CS GPIO belongs to
before and after a transaction, in line with the SPI drivers pm
action hook being called.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>