mailbox: imx: restructure code to make easy for new MU

Add imx_mu_generic_tx for data send and imx_mu_generic_rx for interrupt
data receive.

Pack original mu chans related code into imx_mu_init_generic

Add tx/rx/init hooks into imx_mu_dcfg

With these, it will be a bit easy to introduce i.MX8/8X SCU type
MU dedicated to communicate with SCU.

Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>