sh-pfc: Replace first_gpio and last_gpio with nr_gpios

The SoC information first_gpio field is always equal to 0, and the
last_gpio field is the index of the last entry in the pinmux_gpios
array. Replace the first_gpio and last_gpio fields by a nr_gpios field,
and initialize it to ARRAY_SIZE(pinmux_gpios).

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>