powerpc/powernv: Fix OPAL debug entry

OPAL provides the firmware base/entry in registers at boot time
for debugging purposes. We had a bug in the code trying to stash
these into the appropriate kernel globals (a line of code was
probably dropped by accident back when this was merged)
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/head_64.S

@@ -703,6 +703,7 @@ _INIT_STATIC(start_here_multiplatform)
 
 #ifdef CONFIG_PPC_EARLY_DEBUG_OPAL
 	/* Setup OPAL entry */
+	LOAD_REG_ADDR(r11, opal)
 	std	r28,0(r11);
 	std	r29,8(r11);
 #endif