sunxi: non-FEL SPL boot support for sun7i

Add support for booting from an MMC card.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Henrik Nordström <henrik@henriknordstrom.net>
Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
Reviewed-by: Marek Vasut <marex@denx.de>
Cc: Tom Cubie <Mr.hipboi@gmail.com>
Reviewed-by: Tom Rini <trini@ti.com>