The sunxi-common.h configs header used to contain a lot of random
Allwinner platform related constants, but over the years we moved a lot
of those definitions out there.

Clean up the file to remove outdated comments which are leftovers from
the olden days. Also remove the definition of LOW_LEVEL_SRAM_STACK,
which is actually used nowhere in the whole source tree.

This also uses the opportunity to add some section comments that helps
structuring the header file and improving readability.

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