mailbox: imx: add xSR/xCR register array

We are going to add a new platform which has 4 status registers(SR, TSR,
RSR, GSR) and 4 control registers(CR, TCR, RCR, GCR), so extend xSR
and xCR to register array and adapt code to use it.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>