pinctrl: renesas: r8a7796: 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/81f3586749bb1117c5636e9a9663d25e77cbe158.1649865241.git.geert+renesas@glider.be