[POWERPC] XilinxFB: Add support for custom screen resolution

Some custom implementations of the xilinx fb can use resolutions other
than 640x480.  This patch allows the resolution to be specified in the
device tree or the xilinx_platform_data structure.

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