gpi: Evaluate EE GSI registers offset for QUP

The GSI EE registers offset may vary for some targets based
on the GSI HW version. If so, read the variable offset for
these registers from dt node property.

Change-Id: I585d56552eddc5e6343ebacdc8c004a2f7303d4e
Signed-off-by: Vipin Deep Kaur <vkaur@codeaurora.org>