The FB address is generated by the firmware running on the GPU/VideoCore
and is a "bus" address. This address is the other side of an IOMMU from
the ARM upon which U-Boot is running. Use bus_to_phys() to convert this
to an ARM physical address. Without this, U-Boot seems to work just fine,
but once the Linux kernel boots on an RPi2, accessing the FB memory often
causes a hard system hang.

This is related to:
122426d4 ARM: bcm2835: use phys_to_bus() for mbox
5c0beb5c usb: dwc2: use phys_to_bus/bus_to_phys
79340db7 ARM: bcm2835: implement phys_to_bus/bus_to_phys

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>