remoteproc: k3-m4: Ping the mbox while acquiring the channel

remoteproc: k3-m4: Ping the mbox while acquiring the channel

The TI K3 M4 remoteproc driver acquires the mailbox channel in probe but
sends a message through the acquired channel later in .attach()/.start()
callbacks.

Put both the things together in the form of 'k3_m4_rproc_request_mbox()'
function and invoke that in the probe routine. This is done to align the
rproc_request_mbox() implementation with R5 and DSP drivers which can be
factored out at a later stage.

Signed-off-by: Beleswar Padhi <b-padhi@ti.com>
Tested-by: Judith Mendez <jm@ti.com>
Reviewed-by: Andrew Davis <afd@ti.com>
Link: https://lore.kernel.org/r/20250513054510.3439842-22-b-padhi@ti.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>