mailbox: imx: support channel type tx doorbell v2

The Message Unit(MU) General Purpose Control registers are used for
TX doorbell, but there is no hardware ACK support.

The current TX doorbell channel is using tasklet to emulate hardware
ACK support to kick the TX tick from controller driver side.

The new added TX doorbell channel V2 not using tasklet to emulate the
hardware ACK support. The behavior for the channel is just writing the
GCR register, and no else. This will be used for SCMI mailbox.

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