omap3|4: mux: make local structures static

Mux data is passed by pointers to mux.c from the SoC specific
mux file, these variables dont really need to be global scope.

This fixes the following sparse warnings:
arch/arm/mach-omap2/mux44xx.c:547:29: warning: symbol 'omap4_core_cbl_ball' was not declared. Should it be static?
arch/arm/mach-omap2/mux44xx.c:1265:29: warning: symbol 'omap4_core_cbs_ball' was not declared. Should it be static?
arch/arm/mach-omap2/mux44xx.c:1549:29: warning: symbol 'omap4_wkup_cbl_cbs_ball' was not declared. Should it be static?

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>