[POWERPC] XilinxFB: Allow fixed framebuffer base address

[POWERPC] XilinxFB: Allow fixed framebuffer base address

Allow a fixed framebuffer address to be assigned to the framebuffer device
instead of allocating the framebuffer from the consistent memory pool.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Paul Mackerras <paulus@samba.org>