ARC: HAPS: add HIGHMEM memory zone to DTS

This is required as a preparation of merging nSIM and HASP
defonfig and device tree.

As we have HIGHMEM disabled in both HAPS and nSIM defconfigs
this doesn't lead to any functional change.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>