ARC: Optimize bulk operations on AUX registers

When saving/restoring context auxiliary registers are not sequential so
addresses have to be set explicitly. However still there are some register
that are sequential and this patch exploits the possibility to avoid setting
those address when it is not required. This reduces time required to
write/read auxiliary registers in bulk by about 200 ms.

Signed-off-by: Anton Kolesov <akolesov@synopsys.com>