MMC: omap_hsmmc.c: Add missing prototype header

Add missing header file to fix compilation warning

omap_hsmmc.c: In function 'omap_mmc_init':
omap_hsmmc.c:474: warning: implicit declaration of function 'get_cpu_family'
omap_hsmmc.c:474: warning: implicit declaration of function 'get_cpu_rev'

introduced by commit "MMC: omap_hsmmc.c: disable
multiblock rw on old rev omap34xx silicon"
(4ca9244d)

Signed-off-by: Dirk Behme <dirk.behme@googlemail.com>
CC: Andy Fleming <afleming@freescale.com>
CC: John Rigby <john.rigby@linaro.org>