pinctrl: renesas: Add shorthand for reserved register fields

Currently, reserved register fields must be fully described using dummy
enum IDs (zeroes), one for each possible state (2^bits states).

Add support for describing reserved fields using negative field width
values as shorthands, thus removing the need for dummy values.  Apart
from the obvious size reduction due to the removal of the dummy values,
this will also enable merging adjacent reserved fields into a single
field, reducing the number of fields to describe, and thus kernel size.

Update the checker accordingly.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/cad7c92ef039d9a4d039807efc15886a7aa862be.1649865241.git.geert+renesas@glider.be