MIPS: MTX1: Fix build.

  CC      arch/mips/alchemy/mtx-1/board_setup.o
{standard input}: Assembler messages:
{standard input}:263: Error: opcode not supported on this processor: mips1 (mips1) `sync'
{standard input}:274: Error: opcode not supported on this processor: mips1 (mips1) `sync'
{standard input}:296: Error: opcode not supported on this processor: mips1 (mips1) `sync'
[...]

Any .set mipsX statement other than .set mips0 at the end of inline
assembler is a big fat bug.

Introduced by 9482eabeca315c0276ffb50026b7482481b7097b (linux-mips.org) rsp.
32fd6901 (kernel.org).

While at it, fix the same issue in

  arch/mips/alchemy/devboards/pb1000/board_setup.c
  arch/mips/alchemy/xxs1500/board_setup.c

as well.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>