MIPS: Introduce isa-rev.h to define MIPS_ISA_REV

There are multiple instances in the kernel where we need to include or
exclude particular instructions based on the ISA revision of the target
processor. For MIPS32 / MIPS64, the compiler exports a __mips_isa_rev
define. However, when targeting MIPS I - V, this define is absent. This
leads to each use of __mips_isa_rev having to check that it is defined
first. To simplify this, introduce the isa-rev.h header which always
exports MIPS_ISA_REV. The name is changed so as to avoid confusion with
the compiler builtin and to avoid accidentally using the builtin.
MIPS_ISA_REV is defined to the compilers builtin if provided, or 0,
which satisfies all current usages.

Suggested-by: Paul Burton <paul.burton@mips.com>
Signed-off-by: Matt Redfearn <matt.redfearn@mips.com>
Reviewed-by: Maciej W. Rozycki <macro@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/18676/


Signed-off-by: James Hogan <jhogan@kernel.org>