ARM: vexpress: Use FDT data in platform SMP calls

Scan flatten device looking for A5/A9 SCU node and initialize
it using base address in "reg" property. If nothing is found,
assume that there is no special SCU initialization required
and initialize CPUs basing on numbers of "cpu" type devices
in "cpus" node of the Device Tree.

All this happens only if the board was booted with FDT,
otherwise ct_desc callbacks are used.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>