Fix framebuffer compatibility with new RPi firmware. Framebuffer driver
receives video memory address from VideoCore through property mailbox
channel. Older versions of firmware (and the one that is currently part
of sysutils/u-boot-rpi and sysutils/u-boot-rpi2) returned real physical
address, newer one returns VideoCore bus address, so we need to convert
it to actual physical address. this version works with both older and
newer interface.