sunxi: Factor out eGON BROM header description

To be able to easily share the Allwinner eGON BROM header structure
between the tools and the SPL code, move the struct definition into a
separate header file.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jernej Skrabec <jernej.skrabec@siol.net>
Reviewed-by: Samuel Holland <samuel@sholland.org>