ARM: OMAP2+: sparse: add missing static declaration

Add missing static declaration for file local variables.
This fixes sparse warnings of type:

arch/arm/mach-omap2/omap_hwmod_81xx_data.c:491:26: warning: symbol 'dm81xx_alwon_l3_slow__gpmc' was not declared. Should it be static?

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>