xen/arm64: head: Rework and document setup_fixmap()

xen/arm64: head: Rework and document setup_fixmap()

At the moment, the fixmap table is only hooked when earlyprintk is used.
This is fine today because in C land, the fixmap is not used by anyone
until the the boot CPU is switching to the runtime page-tables.

In the future, the boot CPU will not switch between page-tables to
avoid TLB incoherency. Thus, the fixmap table will need to be always
hooked before any use. Let's start doing it now in setup_fixmap().

Lastly, document the behavior and the main registers usage within the
function.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>