MIPS: perf: Add XLP support for hardware perf.

Add support for XLP performance counters register in perf. Update
mips/Kconfig so that perf events can be selected for XLP.

Signed-off-by: Zi Shen Lim <zlim@netlogicmicro.com>
Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Patchwork: http://patchwork.linux-mips.org/patch/4457
Signed-off-by: John Crispin <blogic@openwrt.org>