ARM: sunxi: Document registers in PSCI code

The PSCI CPU_ON code accesses quite a few registers. Document
their names to make it easier to cross reference.

Also explain "lock cpu" and "unlock cpu" as enabling/disabling
debug access.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>