vexpress64: Enable OF_CONTROL and OF_BOARD for VExpress64

Capture x0 in lowlevel_init.S as potential fdt address. Modify
board_fdt_blob_setup to use fdt address from either vexpress_aemv8.h
or lowlevel_init.S.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>