pinctrl: renesas: r8a77470: Use fixed-width description for IPSR regs

All fields in the IPSR registers on RZ/G1C have the same width, but the
driver describes them using the PINMUX_CFG_REG_VAR() macro, which
is intended for fields with different widths.  Convert the description
to use the PINMUX_CFG_REG() macro for fixed-width fields instead.

This reduces kernel size by 162 bytes.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/f6f26a0dfd16050ead83daf2b9fabeb8b26821a6.1649865241.git.geert+renesas@glider.be