ARM: dts: bcm2711: Move emmc2 into its own bus

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