Fixup register handling in aarch32 reset_handler

The BL handover interface stores the bootloader arguments in
registers r9-r12, so when the reset_handler stores the lr pointer
in r10 it clobers one of the arguments.

Adapt to use r8 and adapt the comment about registers allowed
to clober.

I've checked aarch32 reset_handlers and none seem to use higher
registers as far as I can tell.

Fixes: a6f340fe ("Introduce the new BL handover interface")
Cc: Soby Mathew <soby.mathew@arm.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>