Shrink sh_pfc_pin_config from 8 to 2 bytes:
  - The mux_set flag can be removed, as a non-zero mark value means the
    same (zero = PINMUX_RESERVED is an invalid mark value),
  - The gpio_enabled flag needs only a single bit,
  - Mark values are small integers, and can easily fit in a 15-bit
    bitfield.

This saves 6 bytes per pin when allocating the sh_pfc_pinctrl.configs
array, i.e. it reduces run-time memory consumption by ca. 1.5 KiB.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20201028151637.1734130-5-geert+renesas@glider.be