xen/arm32: head: Rework and document launch()

xen/arm32: head: Rework and document launch()

Boot CPU and secondary CPUs will use different entry point to C code. At
the moment, the decision on which entry to use is taken within launch().

In order to avoid using conditional instruction and make the call
clearer, launch() is reworked to take in parameters the entry point and its
arguments.

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

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