Add uboot "fdt_high" enviroment variable

Add a new "fdt_high" enviroment variable. This can be used to control (or prevent) the
relocation of the flattened device tree on boot. It can be used to prevent relocation
of the fdt into highmem.  The variable behaves similarly to the existing "initrd_high"
variable.

Signed-off-by: David A. Long <dave.long@linaro.org>
diff --git a/README b/README
index 1e2d4d3..6a601d2 100644
--- a/README
+++ b/README
@@ -3305,6 +3305,15 @@
 		  This can be used to load and uncompress arbitrary
 		  data.
 
+  fdt_high	- if set this restricts the maximum address that the
+		  flattened device tree will be copied into upon boot.
+		  If this is set to the special value 0xFFFFFFFF then
+		  the fdt will not be copied at all on boot.  For this
+		  to work it must reside in writable memory, have
+		  sufficient padding on the end of it for u-boot to
+		  add the information it needs into it, and the memory
+		  must be accessible by the kernel.
+
   i2cfast	- (PPC405GP|PPC405EP only)
 		  if set to 'y' configures Linux I2C driver for fast
 		  mode (400kHZ). This environment variable is used in