am335x: Add some code for manipulating GPIOs

Add code for manipulating the GPIOs on the am335x. The API is patterned after
the one used for the Exynos SOCs.

Change-Id: I275317304bd0682f348f72f1c77ed5613065af3f
Signed-off-by: Gabe Black <gabeblack@chromium.org>
Reviewed-on: https://review.coreboot.org/3942
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>
3 files changed