toolchain/toolchain-external-codescape-img-mips: bump to 2018.09-02

toolchain/toolchain-external-codescape-img-mips: bump to 2018.09-02

The 2016.05-06 toolchain we've had support for is pretty outdated at
this point, so update to the latest 2018.09-02 version.

Of note besides the typical component version bumps:

 - The toolchains are now provided by MIPS Tech LLC after its departure
   from Imagination Technologies.

 - The download site changed as a result of that.

 - The toolchains are now built targeting CentOS 6 rather than CentOS 5.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>