ARM: arm1176: enable instruction cache in arch_cpu_init()

Note that this affects all users of the ARM1176 CPU that enable
CONFIG_ARCH_CPU_INIT, not just the BCM2835 SoC, potentially such as
tnetv107x.

Cc: Cyril Chemparathy <cyril@ti.com>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>