fbdev: Add PLB support and cleanup DCR in xilinxfb driver.

Added support for the new xps tft controller. The new core
has PLB interface support in addition to existing DCR interface.

Removed platform device support as both MicroBlaze and PowerPC
use device tree.

Previously, the dcr interface was assumed to be used in mmio mode,
and the register space of the dcr interface was precomputed and stuffed
into the device tree. This driver now makes use of the new dcr
infrastructure to represent the dcr interface. This enables the dcr
interface to be connected directly to a native dcr interface in a clean
way.

Added compatibility for ml507 dvi core.
Signed-off-by: Suneel <suneelg@xilinx.com>
Signed-off-by: Stephen Neuendorffer <stephen.neuendorffer@xilinx.com>
Signed-off-by: John Linn <john.linn@xilinx.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Krzysztof Helt <krzysztof.h1@wp.pl>