include: dts: nrf: add EXMIF pinctrl definitions

Added EXMIP pinctrl definitions, which allow selecting pin functions by
name in DTS. The definitions are added, but not used in pinctrl_nrf.c.

The nrf-regtool reads the pinctrl configuration and applies the settings
using different mechanisms.

Signed-off-by: Rafał Kuźnia <rafal.kuznia@nordicsemi.no>