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>

ARC: add helpers to sanitize config options

We'll use this macro in coming patches extensively.

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