opcodes: Correct address for ARC's "isa_config" aux reg

This patch changes the address for "isa_config" auxiliary register
from 0xC2 to the correct value 0xC1.  Moreover, it only exists in
arc700+ and not all ARCs.

opcodes/ChangeLog:

	* arc-regs.h: Change isa_config address to 0xc1.
	isa_config exists for ARC700 and ARCV2 and not ARCALL.