ARM: omap2+: mux: Allow board mux settings to be NULL

OMAP4 has two mux instances, and the board may not have settings
for one of them.  Allow the board file to pass NULL for an
instance's mux settings, which will initialize the mux instance
but skip writing board settings.

Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>