ARC: Optimize writes and reads of registers

When saving/restoring context all core and auxiliary registers are
read/written. This patch serializes those actions, reducing amount of
JTAG operations, and thus reducing required time. Previously each
register was written in a separate JTAG transaction and whole operation
on context was taking around 2100 ms. After this patch it takes around
600-700 ms to finish. This is still too slow but at least 3 times better
then before. Further optimizations in this area might be researched and
implemented.

Signed-off-by: Anton Kolesov <akolesov@synopsys.com>