MLK-10078-02 arm: mcc: platform related codes changes

The platform related codes changes when enable mcc2.0
on imx_3.14 kernel, and tested on imx6sx sdb board.
- keep imx6sx soc related apis in mcc_imx6sx.c/h
- keep linux os related apis in mcc_linux.c/h
- add some new mcc callback in mu driver, since the
gie3 of mu is used as cpu2cpu interrupter in the mcc
implementation on imx6sx.

Signed-off-by: Richard Zhu <Richard.Zhu@freescale.com>