omap: mux: Do keypad muxing in board-*.c files

This way we get pin muxing out of plat-omap and can convert H4 to use the
new mux functions.

Note that it should be safe to assume we can mux all the keypad pins
except on H4 which may have Menelaus connected.

Signed-off-by: Tony Lindgren <tony@atomide.com>