sunxi: Kconfig: use SoC-wide values for some symbols

sunxi: Kconfig: use SoC-wide values for some symbols

Some configuration symbols formerly defined in header files were
recently converted to Kconfig symbols. This moved their value definition
into *every* defconfig file, even though those values are hardly board
choices.
Use the new Kconfig option to define per-SoC default values, in just one
place, which makes the definition in each defconfig file redundant.

We refrain from setting a sunxi specific value for CONFIG_SYS_BOOTM_LEN,
so this defaults to a much better 64MB for uncompressed arm64 kernels.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Tom Rini <trini@konsulko.com>