arm: mach-omap2: remove NULL board_mux from board files

If CONFIG_OMAP_MUX is not enabled, we can define board_mux in the header
file instead of forcing every single board to define it.

Signed-off-by: Aaro Koskinen <aaro.koskinen@nokia.com>
[tony@atomide.com: updated for combined board-zoom files]
Signed-off-by: Tony Lindgren <tony@atomide.com>