arm: mach-omap2: cache: Explicitly enable I cache

omap-common cache enabling sequence relies on cpu_init_cp15()
(inside start.S) for enabling I-caches. But cpu_init_cp15()
can be skipped if CONFIG_SKIP_LOWLEVEL_INIT is defined. So
enable I-caches if not enabled already.

Debugged-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Tested-by: Steve Kipisz <s-kipisz2@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Tested-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>