arm: omap1: mbox: make variables static

Make some variables static to get rid of the following warnings:

arch/arm/mach-omap1/mailbox.c:136:18: warning: symbol 'mbox_dsp_info' was not declared. Should it be static?
arch/arm/mach-omap1/mailbox.c:142:18: warning: symbol 'omap1_mboxes' was not declared. Should it be static?

Signed-off-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>