PPC: Use r2 instead of r29 as global data pointer

R29 was an unlucky choice as with recent toolchains (gcc-4.2.x) gcc
will refuse to use load/store multiple insns; instead, it issues a
list of simple load/store instructions upon function entry and exit,
resulting in bigger code size, which in turn makes the build for a
few boards fail.

Use r2 instead.

Signed-off-by: Wolfgang Denk <wd@denx.de>
diff --git a/README b/README
index a5ffbb5..b4966a0 100644
--- a/README
+++ b/README
@@ -3499,7 +3499,7 @@
 
 For PowerPC, the following registers have specific use:
 	R1:	stack pointer
-	R2:	TOC pointer
+	R2:	reserved for system use
 	R3-R4:	parameter passing and return values
 	R5-R10: parameter passing
 	R13:	small data area pointer
@@ -3508,7 +3508,7 @@
 
 	(U-Boot also uses R14 as internal GOT pointer.)
 
-    ==> U-Boot will use R29 to hold a pointer to the global data
+    ==> U-Boot will use R2 to hold a pointer to the global data
 
     Note: on PPC, we could use a static initializer (since the
     address of the global data structure is known at compile time),