arch-mips: Add mipsisa{32, 64}r6{el, } tunes

arch-mips: Add mipsisa{32, 64}r6{el, } tunes

Add support for MIPS Release 6 ISA and the various tune
configurations.

This patch adds the tunes for 32r6 and 64r6 n64 and not the n32
variants at the moment.

Release 6 onwards, the tuples are now
 - mipsisa32r6-linux-gnu
 - mipsisa32r6el-linux-gnu
 - mipsisa64r6-linux-gnuabi64
 - mipsisa64r6el-linux-gnuabi64
 - mipsisa64r6-linux-gnuabin32
 - mipsisa64r6el-linux-gnuabin32

For more details, check https://wiki.debian.org/Multiarch/Tuples

Signed-off-by: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>