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 81 bytes.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/ecc7377d2992694226dcf055bed0b617701a3d71.1649865241.git.geert+renesas@glider.be