Current mailbox is blocking by default, but there are cases where we don't
need to wait for a response.

Linux just needs to send data to the remote processor, so let's
allow tx_block mode to be set (true/false) depending on usecase.

No functional changes.
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Link: https://lore.kernel.org/r/20240822-imx_rproc-v3-1-6d943723945d@nxp.comSigned-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>