Fix BL2U entrypoint possible parameter corruption

Replace the use of r12 by r10 to save the value of a parameter of
bl2u_entrypoint to pass it to bl2u_early_platform_setup at the end of
the function. r10 is a callee saved register so it will not become
corrupted by C code, whereas r12 is the The Intra-Procedure-call scratch
register potentially used by veneers. See the ARM AAPCS document (ARM
IHI 0042F).

Change-Id: I4f37e54a6b550719edb40bb24cd8f498827e2749
Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>