MIPS: SmartMIPS: Disable assembler warnings

MIPS: SmartMIPS: Disable assembler warnings

The kernel code overrides the default ISA as passed by the compiler
in quite a few places. This has unfortunate side effects when smartmips
is enabled leading to hundreds of warnings during build such as:

{standard input}: Assembler messages:
{standard input}:411: Warning: the `smartmips' extension requires MIPS32
revision 1 or greater
{standard input}: Assembler messages:
{standard input}:43: Warning: the 64-bit MIPS architecture does not support the
`smartmips' extension
[...]

Until the kernel code is fixed properly (if possible), disable all the
assembler warning messages to make the build logs readable again.
This has no runtime side effects but it makes it easier to spot
more critical warnings and problems during build.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/7356/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>