powerpc/6xx: Don't use SPRN_SPRG2 for storing stack pointer while in RTAS

When calling RTAS, the stack pointer is stored in SPRN_SPRG2
in order to be able to restore it in case of machine check in RTAS.

As machine check is not a perfomance critical path, this patch
frees SPRN_SPRG2 by using a field in thread struct instead.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/processor.h

@@ -250,6 +250,9 @@ struct thread_struct {
 #ifdef CONFIG_PPC32
 	void		*pgdir;		/* root of page-table tree */
 	unsigned long	ksp_limit;	/* if ksp <= ksp_limit stack overflow */
+#ifdef CONFIG_PPC_RTAS
+	unsigned long	rtas_sp;	/* stack pointer for when in RTAS */
+#endif
 #endif
 	/* Debug Registers */
 	struct debug_reg debug;