sunxi: simplify U-Boot proper only builds

sunxi: simplify U-Boot proper only builds

At the moment every Allwinner board builds and requires an SPL, even
though we select this individually in each _defconfig file.
For experiments and for early bringup of new SoCs it would be beneficial
to only build U-Boot proper, for instance to postpone a tedious SPL port
(including DRAM support) in the initial phase.

Protect some SPL related symbols that we unconditionally select at the
moment with "if SPL", to avoid Kconfig conflicts when CONFIG_SPL is
disabled.

This alone does not cleanly build U-Boot proper only yet, but gets it
far enough so that the binary can be harvested.

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