powerpc/fsl: Fix mfpmr build errors with newer binutils

powerpc/fsl: Fix mfpmr build errors with newer binutils

Binutils 2.38 complains about the use of mfpmr when building
ppc6xx_defconfig:

    CC      arch/powerpc/kernel/pmc.o
  {standard input}: Assembler messages:
  {standard input}:45: Error: unrecognized opcode: `mfpmr'
  {standard input}:56: Error: unrecognized opcode: `mtpmr'

This is because by default the kernel is built with -mcpu=powerpc, and
the mt/mfpmr instructions are not defined.

It can be avoided by enabling CONFIG_E300C3_CPU, but just adding that to
the defconfig will leave open the possibility of randconfig failures.

So add machine directives around the mt/mfpmr instructions to tell
binutils how to assemble them.

Cc: stable@vger.kernel.org
Reported-by: Jan-Benedict Glaw <jbglaw@lug-owl.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240229122521.762431-3-mpe@ellerman.id.au