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>

arch/arm/mach-omap2/board-am3517evm.c

@@ -442,8 +442,6 @@ static struct omap_board_mux board_mux[] __initdata = {
 	OMAP3_MUX(SAD2D_MCAD23, OMAP_MUX_MODE0 | OMAP_PIN_INPUT_PULLDOWN),
 	{ .reg_offset = OMAP_MUX_TERMINATOR },
 };
-#else
-#define board_mux	NULL
 #endif