powerpc/mpc8xxx: Fix TIMING_CFG_3[EXT_ACTTOPRE]

The TIMING_CFG_3[EXT_ACTTOPRE] register field is 2 bits wide, but
the mask omitted the LSB.  This patch provides a 2-bit wide mask.

Signed-off-by: James Yang <James.Yang@freescale.com>
Signed-off-by: York Sun <yorksun@freescale.com>

arch/powerpc/cpu/mpc8xxx/ddr/ctrl_regs.c

@@ -364,7 +364,7 @@ static void set_timing_cfg_3(fsl_ddr_cfg_regs_t *ddr,
 
 	ddr->timing_cfg_3 = (0
 		| ((ext_pretoact & 0x1) << 28)
-		| ((ext_acttopre & 0x2) << 24)
+		| ((ext_acttopre & 0x3) << 24)
 		| ((ext_acttorw & 0x1) << 22)
 		| ((ext_refrec & 0x1F) << 16)
 		| ((ext_caslat & 0x3) << 12)