powerpc/32: Use fast instruction to set MSR RI in exception prolog on 8xx

8xx has registers SPRN_NRI, SPRN_EID and SPRN_EIE for changing
MSR EE and RI.

Use SPRN_EID in exception prolog to set RI.

On an 8xx, it reduces the null_syscall test by 3 cycles.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/65f6bda827c2a2abce71ea7e07543e791163da33.1615552866.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/head_32.h

@@ -107,6 +107,8 @@
 #endif
 #ifdef CONFIG_40x
 	rlwinm	r9,r9,0,14,12		/* clear MSR_WE (necessary?) */
+#elif defined(CONFIG_PPC_8xx)
+	mtspr	SPRN_EID, r2		/* Set MSR_RI */
 #else
 #ifdef CONFIG_VMAP_STACK
 	li	r10, MSR_KERNEL & ~MSR_IR /* can take exceptions */