gpio: For EM Starterkit, some additional GPIO register defines are needed

Comparing the Synopsys GPIO driver with the one here in Zephyr revealed
that some register symbols are missing. I am adding them now, and will
be working on getting GPIO working on ARC EM Starterkit board.

Change-Id: Ifccd1e225eb1373a31c6a5c51cf3927e42601d1a
Signed-off-by: Chuck Jordan <cjordan@synopsys.com>