dts: arm: xilinx: zynqmp: add OCM memory area

Add the 256 kB On-Chip memory area which is located at 0xfffc0000.
In conjunction with a corresponding 'chosen' entry at the board level,
equivalent to board device trees based on the Zynq-7000, explicit placement
of data in the OCM becomes possible, for example when setting up the GEM
Ethernet Controller's DMA area.

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