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 70 bytes.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/c5183fcb3dd417d57ced0f60d091e2c7d37e1c8c.1649865241.git.geert+renesas@glider.be