arc: add XY mem support

The XY Memory is a feature commonly found in DSP processors to increase
the DSP performance. The XY component allows a ARC processor to
implicitly load source operands and store results into a closely coupled
memory using a single instruction.

Add XY memory for ARC EM9D/EM11D processors including em_starterkit,
em_starterkit_em11d. emsdp_em9d, nsim_em, iotdk.

Signed-off-by: Siyuan Cheng <siyuanc@synopsys.com>