drivers: spi: nrfx: add dependency to PPI for PAN 58 on nRF52832

While enabling workaround for PAN 58 the PPI driver is used.
This requires the nrfx PPI driver to be enabled thus CONFIG_NRFX_PPI
Kconfig symbol needs to be set.

Jira: NRFX-1616

Signed-off-by: Marcin Szymczyk <marcin.szymczyk@nordicsemi.no>