mailbox: imx: replace the xTR/xRR array with single register

The xTR/xRR registers are using 4 bytes stride and continuous.
Considering we will support more TR and RR registers, use base + idx * 4
method to calculate register address, not hardcoding in driver.

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