valgrind: disable for mips64r6/mips64elr6

Fixes [1]:

  /tmp/ccBAUqbI.s: Assembler messages:
  /tmp/ccBAUqbI.s:622: Error: opcode not supported on this processor: mips64r6 (mips64r6) `movn $2,$4,$7'
  /tmp/ccBAUqbI.s:2607: Error: opcode not supported on this processor: mips64r6 (mips64r6) `movn $2,$4,$7'

[1] http://autobuild.buildroot.net/results/dc52088222e9fbedcebffc1c39be6d2fecfffe5d



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