U-Boot's generic GPIO_EXTRA_HEADER is a convenience symbol to allow code
to more easily include platform specific GPIO headers. This should not
be needed in a DM world anymore, since the generic GPIO framework
handles that nicely.
For Allwinner boards we still need to deal with non-DM GPIO in the SPL,
but this should become the exception, not the rule.

Make this more obvious by removing the definition of GPIO_EXTRA_HEADER,
and just force every legacy user of platform specific GPIO to include
the new sunxi_gpio.h header explicitly. Everyone doing so should feel
ashamed and should find a way to avoid it from now on.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Samuel Holland <samuel@sholland.org>