ARCv2: [plat-hsdk]: Save accl reg pair by default

This manifsted as strace segfaulting on HSDK because gcc was targetting
the accumulator registers as GPRs, which kernek was not saving/restoring
by default.

Cc: stable@vger.kernel.org   #4.14+
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>