MIPS: Introduce _EXT assembler macro

This patch adds a simple macro to wrap the ext instruction which was
introduced with MIPSR2, and fall back to a shift & and pair for
pre-MIPSR2 CPUs. This will be used in a subsequent patch.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6358/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>