Fix the sc(4) framebuffer driver on RPi 2.

Use the BCM2835_MBOX_CHAN_PROP mbox channel to setup the framebuffer,
remove unused code and unnecessary includes.

Adjust the color palette when bcm2708_fb.fbswap is set on /chosen/bootargs
node of DTB. The firmware used on RPi 2 uses this mode.

Tested on: RPi-B and RPi 2 with 16, 24 and 32bpp