sh: update PFC to allow any enum in MARK lists

This patch updates the PFC code with some clarifying
comments together with a functional change. The change
allows function type of GPIO to select any type of enum
in their MARK lists. Without this patch only function
type of enums are allowed in MARK lists.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>