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/examples/test_burst_lib.S b/examples/test_burst_lib.S
index 5bb4981..aef4e32 100644
--- a/examples/test_burst_lib.S
+++ b/examples/test_burst_lib.S
@@ -70,7 +70,7 @@
 	 * we can load the instruction and data TLB registers with the
 	 * same values.
 	 */
-	lwz	r9,20(r29)		/* gd->ram_size */
+	lwz	r9,20(r2)		/* gd->ram_size */
 	addis	r9,r9,-0x80
 
 	mr	r8, r9			/* Higher 8 Meg in SDRAM */