drivers: spi: nrfx: Add explicit dependency to GPIO

This dependency was always there but not explicitly defined.
By adding the explicit dependency it becomes more obvious
what is wrong when SPI is enabled but GPIO disabled.

This was found when building `samples/bluetooth/peripheral`
for `nrf54l15dk/nrf54l15/cpuapp` with `CONFIG_GPIO=n`.

Before we got:
 - A linker error in `spi_nrfx_common.c` failing
   to reference some nrfx_gpiote APIs.
 - A linker error in `spi_nrfx_spim.c` failing to reference
   the GPIO dts entry.

Now we will get a warning of that GPIO is not enabled
With this it becomes more obvious that SPI driver is enabled by
default because of the external flash mounted on the DK.

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>