pinctrl: renesas: r8a7796: Use shorthands for reserved fields

Replace the full descriptions of reserved register fields by shorthands
with a negative field width, and merge adjacent reserved fields.

This reduces kernel size by 148 bytes.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/433f5ddcc2dba7352825cba007b99b8e654d4c61.1649865241.git.geert+renesas@glider.be