sunxi-common.h: remove pointless #ifdefs

Remove some pointless #ifdefs from this file, as there are quite too
many of them already.

Some definitions don't really hurt to have in any case, so remove the
pointless CONFIG_MMC guard around CONFIG_MMC_SUNXI_SLOT.

The BOARD_SIZE_LIMIT applies regardless of ARM64 or not (now), so remove
that guard as well. The maximum number of MMC devices does not depend on
CONFIG_ENV_IS_IN_MMC, so move that out to simplify the file.

Last but not least CONFIG_SPL_BOARD_LOAD_IMAGE serves no real purpose
anymore: it's unconditionally defined for all sunxi boards, and protects
nothing applicable outside of sunxi code anymore. Just remove it.

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