pinctrl: sh-pfc: r8a77980: Add missing MOD_SEL0 field

The Module Select Register 0 contains 20 (= 5 x 4) reserved bits, and 12
single-bit fields, but the variable field descriptor lacks a field of 4
reserved bits.

Fixes: f5912524 ("pinctrl: sh-pfc: Add R8A77980 PFC support")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>