arm: sunxi: Enable SPL/SPI boot for Olinuxino Lime2-eMMC boards

Starting from revision K the Olimex-Lime2-eMMC boards include
eMMC 5.x chip. Unfortunately, the internal brom in the A20
cannot reliably initialize those chips resulting in random
boot failures on those boards.

To overcome this, all the latest Lime2-eMMC boards are
populated with 16MB SPI flash and therefore enable support
for SPI SPL boot.

For now, SPI flash can be managed using sunxi-fel's spiflash tools.

Signed-off-by: Priit Laes <priit.laes@paf.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>