ARC: [plat-eznps] Add missing struct nps_host_reg_aux_dpc

Fixing compilation issue caused by missing struct nps_host_reg_aux_dpc
definition.

Fixes: 3f9cd874 ("ARC: [plat-eznps] avoid toggling of DPC register")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Ofer Levi <oferle@mellanox.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>