sunxi: remove unused CONFIG_MMC_SUNXI_SLOT

There is a CONFIG_MMC_SUNXI_SLOT definition in our sunxi_common.h config
header, which was used to note the first MMC controller to initialise.
The definition in that header was always set to 0, with no easy way of
overriding this, and certainly none of the existing boards made any use
of that (non-)feature.
Remove that definition and replace it with a constant 0 in the only
user, in board.c. It turns out that this is safe, as this is only used
in the SPL, and the BROM also unconditionally initialises MMC0.
This also removes the last legacy config symbol with SUN*I in it from
the whitelist.

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