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 257 bytes.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/0f211d493a0cfbcd96d84a709d21bea51c7385ae.1649865241.git.geert+renesas@glider.be