Add support for mainboard specific suspend/resume handler

Some mainboards (most likely laptops) will need mainboard specific functions
called upon a resume from suspend.

Change-Id: If1518a4b016bba776643adaef0ae64ff49f57e51
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/852
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>