sunxi: A64: drop boot0 header reservation

In the early days of the Allwinner A64 U-Boot support, we relied on a
vendor provided "boot0" binary to perform the DRAM initialisation. This
replaced the SPL, and required to equip the U-Boot (proper) binary with
a vendor specific header to be recognised as a valid boot0 payload.
Fortunately these days are long gone (we gained SPL and DRAM support in
early 2017!), and we never needed to use that hack on any later 64-bit
Allwinner SoC.

Since this is highly obsolete by now, remove that option from the
defconfigs of all A64 boards. We leave the code still in here for now,
since some people expressed their interest in this.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>