[POWERPC] Xilinx: Framebuffer: Use dcr infrastructure.

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

In particular, the device tree expected looks like:

			dcr_v29_0: dcr@0 {
				#address-cells = <1>;
				#size-cells = <1>;
				compatible = "xlnx,dcr-v29-1.00.a";
				VGA_FrameBuffer: tft@80 {
					compatible = "xlnx,plb-tft-cntlr-ref-1.00.a";
					dcr-parent = <&opb2dcr_bridge_0>;
					dcr-reg = < 80 2 >;
					xlnx,default-tft-base-addr = <7f>;
					xlnx,dps-init = <1>;
					xlnx,on-init = <1>;
					xlnx,pixclk-is-busclk-divby4 = <1>;
				} ;
			} ;

			opb2dcr_bridge_0: opb2dcr-bridge@40700000 {
				compatible = "xlnx,opb2dcr-bridge-1.00.b";
				dcr-access-method = "mmio";
				dcr-controller ;
				dcr-mmio-range = < 40700000 1000 >;
				dcr-mmio-stride = <4>;
				reg = < 40700000 1000 >;
				xlnx,family = "virtex2p";
			} ;

Signed-off-by: Stephen Neuendorffer <stephen.neuendorffer@xilinx.com>