fdt: ARM: Add fdtcontroladdr to set device tree address in environment

This adds support for a new environment variable called 'fdtcontroladdr'. If
defined, the hex address is used as the address of the control fdt for U-Boot.

Note: I have not changed CONFIG_PRAM section as I already have an
outstanding patch on that.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/README b/README
index 558f011..a3b62e1 100644
--- a/README
+++ b/README
@@ -3499,6 +3499,10 @@
 		  add the information it needs into it, and the memory
 		  must be accessible by the kernel.
 
+  fdtcontroladdr- if set this is the address of the control flattened
+		  device tree used by U-Boot when CONFIG_OF_CONTROL is
+		  defined.
+
   i2cfast	- (PPC405GP|PPC405EP only)
 		  if set to 'y' configures Linux I2C driver for fast
 		  mode (400kHZ). This environment variable is used in