dts: arm: xilinx: Assign uniqe memory-region names for OCM

Assign unique memory-region attribute values for both OCM
memory areas so that the lower region starting at address 0
is identified as OCM_LOW, while the alternate location for
the OCM memory is identified as OCM_HIGH.

The updated linker command file will use those identifiers
for OCM section mapping based on which of the two OCM areas
is marked as 'chosen' at the board level.

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