pinctrl: renesas: r8a77965: Optimize fixed-width reserved fields

Describe registers with fixed-width register fields and many reserved
fields using the PINMUX_CFG_REG_VAR() macro, as the latter supports a
shorthand not requiring dummy values.

This reduces kernel size by 496 bytes.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/2aff2f4c1ed6d834370ce6dd9379c8c93bfc0a92.1649865241.git.geert+renesas@glider.be