Add code to clean and invalidate caches

Linux expects to be handed a system with caches disabled and
invalidated. While the model currently brings CPUs up with caches
invalidated, we'll need to invalidate caches when leaving the
bootwrapper if we enable them within the bootwrapper (e.g. for the use
of exclusive operations), as lines may be allocated.

This patch adds code to invalidate the dcaches and icaches. It is not
yet called.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>