MIPS: Netlogic: select MIPSR2 for XLP

This allows us to use the r2 optimized code from kernel headers
while compilation.

Disable PGD_C0_CONTEXT option for XLP, which does not work.

Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Patchwork: http://patchwork.linux-mips.org/patch/4456
Signed-off-by: John Crispin <blogic@openwrt.org>