mailbox: imx: add i.MX8 SECO MU support

i.MX8/8X SECO firmware IPC is an implementation of passing messages.
But current imx-mailbox driver only support one word  message,
i.MX8/8X linux side firmware has to request four TX, four RX and a
TXDB to support IPC to SECO firmware. This is low efficent and
more interrupts triggered compared with one TX and one RX.

To make SECO MU work,
  - parse the size of msg.
  - Only enable TR0/RR0 interrupt for transmit/receive message.
  - For TX/RX, only support one TX channel and one RX channel
  - For RX, support receive msg of any size, limited by hardcoded value
    of 30.

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