xen/ppc: Fix stack initialization in head.S

The code to set up the stack in head.S erroneously loads the bottom of
the stack (the symbol cpu0_boot_stack) into r1 instead of the top of the
stack (cpu0_boot_stack + STACK_SIZE).

Fixes: 3a4e6f67bc ("xen/ppc: Set up a basic C environment")
Signed-off-by: Shawn Anastasio <sanastasio@raptorengineering.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>

xen/arch/ppc/ppc64/head.S

@@ -46,7 +46,7 @@ ENTRY(start)
 .L_correct_address:
 
     /* set up the initial stack */
-    LOAD_REG_ADDR(%r1, cpu0_boot_stack)
+    LOAD_REG_ADDR(%r1, cpu0_boot_stack + STACK_SIZE)
     li      %r11, 0
     stdu    %r11, -STACK_FRAME_OVERHEAD(%r1)