[Sound-open-firmware] Linux build for arm64

Hi Liam / Pierre,

Could we add a build for arm64 too in CI? With recent patches that add support for i.MX8 SOF Linux driver it would be handy to also have the build for arm64.

I can help you with the cross-compilers and build instructions.

Currently we are only building for x86_64.

https://travis-ci.org/thesofproject/linux/builds/560509972

thanks, Daniel.