It is fairly common for the pins in a pin group to be a subset of the
pins in another pin group.  Add a macro for defining a pin group that
refers to a subset of an array of pins.  This allows pin groups to share
pin data, and thus save memory.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/b56c4257aee1eab698bae2cf7a08aa05775c0a77.1640269757.git.geert+renesas@glider.be