riscv: start on work to allow big-endian build

[Ben Dooks]

Given we've been doing work on qemu and linux-kernel we started
looking at u-boot too. I've done a quick test with our qemu and
got a few patches going.

It looks like there are only a few updates to get the build
working. The only question I have at the moment is with the
read/write io macros which seem to be incosistently endian
interested.

project at: https://gitlab.com/CodethinkLabs/riscv_bigendian