bcm2835: Add simiple-framebuffer for use with fkms

When the fkms dtb overlay is used only the simple-framebuffer is
presented as a usable video display. So, add "simple-framebuffer"
compatible to enable video driver bcm2835.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Meng Li <Meng.Li@windriver.com>