toolchain-external: add CodeSourcery MIPS 2015.05, remove 2013.11

- Add CodeSourcery MIPS 2015.05 toolchain
- Remove CodeSourcery MIPS 2013.11 toolchain
- Update the hash file

Toolchain datasheet:
  https://sourcery.mentor.com/GNUToolchain/release3068?@template=datasheet



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