sh-pfc: Rename struct sh_pfc nr_pins field to nr_gpio_pins

The field contains the number of pins with an associated GPIO port. This
is currently equal to the total number of pins but will be modified when
adding support for pins without a GPIO port. Rename the field
accordingly.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Yusuke Goda <yusuke.goda.sx@renesas.com>