mmc: Register only the first MMC device on MMC_TINY

When MMC_TINY is enabled, support for only one MMC device
is provided. Boards that register more than one device,
will just write over mmc_static keeping only the last one
registered.

This commit prevents this, keeping only the first MMC
device created. A debug warning message is added, if nothing
else, as a hint/documentation for developers.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>