Removed commented-out code
Fixed initialisation code

- Configure write-back caching in page table. This is needed for
  reliable hypercalls to Xen (thanks to Julien Grall).

- Use "client mode" for access control (domains are deprecated,
  according to ARM Cortex-A Series Programmer’s Guide version 4.0,
  section 9.6.4).

- Enable more SCTLR features (icache, branch prediction).

- Added more comments.

This allows Mini-OS to boot on the Cubieboard 2.