event: Add an event for main_loop

Add an event type EVT_MAIN_LOOP that can be used for registering
events that need to be run after the platform has been initialised and
before the main_loop function is called.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>