sh-pfc: Support pins not associated with a GPIO port

Pins with selectable functions but without a GPIO port can't be named
PORT_# or GP_#_#. Add a SH_PFC_PIN_NAMED macro to declare such pins in
the pinmux pins array, naming them with the PIN_ prefix followed by the
pin physical position.

In order to make sure not to register those pins as GPIOs, add a
SH_PFC_PIN_CFG_NO_GPIO pin flag to denote pins without a GPIO port.

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