ARC: allow userspace DSP applications to use AGU extensions

To be able to run DSP-enabled userspace applications with AGU
(address generation unit) extensions we additionally need to
save and restore following registers at context switch:
 * AGU_AP*
 * AGU_OS*
 * AGU_MOD*

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