valgrind: add a patch to fix MIPS R6 by replacing ADDI with ADDIU

The ADDI instruction has been removed for MIPS R6, so valgrind fails to
build for this revision level because it uses that assembly
instruction. Using ADDIU instead fixes the problem.

The patch has been sent upstream:

  https://bugs.kde.org/show_bug.cgi?id=356112

Fixes:

  http://autobuild.buildroot.net/results/488/488937704b3b6bae8daed7b24da8c740b56f4d84/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>