ARM: mmp: select CPU_PJ4

Since CPU_PJ4 is shared between PXA95x and MMP2, select CPU_PJ4 in MMP2
configuration.

Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>