arm: socfpga: Enable SDMMC boot for SOCFPGA U-Boot

Enable the SDMMC boot as default boot for SOCFPGA U-Boot dev kit.
Enable the bootz command as zImage is used instead uImage.

Signed-off-by: Chin Liang See <clsee@altera.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Dinh Nguyen <dinguyen@altera.com>
Cc: Pavel Machek <pavel@denx.de>
Cc: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@ti.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Wolfgang Denk <wd@denx.de>
Acked-by: Pavel Machek <pavel@denx.de>