[POWERPC] XilinxFB: Use pdata to pass around framebuffer parameters

The call to xilinxfb_assign is getting unwieldy when adding features
to the Xilinx framebuffer driver.  Change xilinxfb_assign() to accept
a pointer to a xilinxfb_platform_data structure to prepare for adding
additition configuration options.

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