pinctrl: renesas: sh7264: 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 572 bytes.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/434c274f626b2eab3539fe2ab80c6eda164e07fa.1649865241.git.geert+renesas@glider.be