framebuffer: Add a framebuffer category of drivers.

These drivers provide a function to fill in a structure describing a
framebuffer. They can either get information about the framebuffer from the
FWDB, or they can derive it themselves by, for instance, initializing the
hardware that provides the framebuffer.

Change-Id: I74e9f3ff8f84c444e02a64840ac865075d1ea076
8 files changed