valgrind: disable for mips32r6/mips32elr6

Fixes [1]:

  /tmp/ccD2Tule.s: Assembler messages:
  /tmp/ccD2Tule.s:682: Error: opcode not supported on this processor: mips32r6 (mips32r6) `movn $2,$4,$7'
  /tmp/ccD2Tule.s:2767: Error: opcode not supported on this processor: mips32r6 (mips32r6) `movn $2,$4,$7'

[1] http://autobuild.buildroot.net/results/f0253d1ed11021d3e5914a5161360de3ef3d1641



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>