ARC: Add missing register names for invisible registers

Registers R61 (reserved) and R62 (limm) are not accessible via JTAG, but they
must be defined and sent to the GDB. arc_gdb_reg_names_list was missing names
for those registers, but still were counted in the ARC32_NUM_GDB_REGS and hold
sites in the arrays of register values user to save/restore context, though
values on those sites were never accessed.

Signed-off-by: Anton Kolesov <akolesov@synopsys.com>