remoteproc: k3: Refactor rproc_request_mbox() implementations into common driver

remoteproc: k3: Refactor rproc_request_mbox() implementations into common driver

The rproc_request_mbox() implementations in TI K3 R5, DSP and M4
remoteproc drivers acquire the mailbox channel and send the same message
through the acquired channel.

Refactor the above function into the ti_k3_common.c driver as
k3_rproc_request_mbox() and use it throughout R5, DSP and M4 drivers.

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/20250513054510.3439842-23-b-padhi@ti.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>