ARM: OMAP2+: Remove legacy mux code

All the boards booting with device tree use
drivers/pinctrl-single.c instead.

Note that mach-omap1 is still using the legacy mux,
so let's move the related Kconfig options from plat-omap
to mach-omap1.

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