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>