openblas: P5600 and I6400 cores only available for Codescape toolchains

openblas: P5600 and I6400 cores only available for Codescape toolchains

These two MIPS cores are built with MSA support (-mmsa) which is
currently in GCC trunk. The only toolchains that support it right now
are Codescape MTI and Codescape IMG from Imagination Technologies.

Fixes:
  http://autobuild.buildroot.net/results/af5/af5e9f10287aaf9b9be6228ad7d12b2d926f8051/
  http://autobuild.buildroot.net/results/b20/b201e4072b0b634d5ac86781aee24c698f88d689/

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>