mediatek: select fip.bin location according to boot partition

As of now we support only MT8183, we should avoid duplicating the code for all boards.

MMC driver provides mmc_current_boot_part() function, which has been exported to use it into bl2_plat_setup.c code.

Choose the next partition in the boot flow as follow:

	mmc0boot0 ---> bootloaders
	mmc0boot1 ---> bootloaders2

This will provide a way to atomically update the bootloaders.

Names must remain in line with the names in Yocto's WIC file.

Signed-off-by: pboos pboos@baylibre.com