Depending on bcm2711's revision its emmc2 controller might have
different DMA constraints. Raspberry Pi 4's firmware will take care of
updating those, but only if a certain alias is found in the device tree.
So, move emmc2 into its own bus, so as not to pollute other devices with
dma-ranges changes and create the emmc2bus alias.

Based in Phil ELwell's downstream implementation.
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Link: https://lore.kernel.org/r/20200304132437.20164-1-nsaenzjulienne@suse.de