ARM: provide a late_initcall hook for platform initialization

This allows platforms to set up things that need to be done at
late_initcall time.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Tested-by: Robert Lee <rob.lee@linaro.org>
Tested-by: Stephen Warren <swarren@wwwdotorg.org>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>