ARC: pt_regs: create seperate type for ecr

Reduces duplication in each ISA specific pt_regs

Tested-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202308151342.ROQ9Urvv-lkp@intel.com


Signed-off-by: Vineet Gupta <vgupta@kernel.org>