RFC: big endian support for riscv64/opensbi

[Ben Dooks]

This is part of our project to investigate big endian on RISC-V.
The full information is documented on our gitlab[1] which includes
source repositories, build information and project documentation.

We have a minimal buildroot, qemu and kernel working on QEMU.

As this is a work in progress any review or help is appreciated.

[1] https://gitlab.com/CodethinkLabs/riscv_bigendian