drivers: xlnx_gem: remove Zynq/ZynqMP distinction for DMA placement

With the unification of OCM declaration & assignment between Zynq-7000
and Ultrascale/ZynqMP, remove the distinction between those two SoC
families so that the DMA area is always set up in the OCM regardless
of the current SoC type.

Signed-off-by: Immo Birnbaum <Immo.Birnbaum@weidmueller.com>