mailbox: Add get_max_txsize() to mbox_chan_ops

mailbox: Add get_max_txsize() to mbox_chan_ops

Add mbox_get_max_txsize() API to the mailbox client API's as
currently there is no provision for the client to get the max
tx message size that can be sent. This API queries the controller
driver and returns the max tx message size.

Bug 200278347

Change-Id: I533f9d137c7eba473b9004ad1000fb85b50c33d7
Signed-off-by: Akhilesh Reddy Khumbum <akhumbum@nvidia.com>
Reviewed-on: https://git-master/r/1506865
Tested-by: Rohit Vaswani <rvaswani@nvidia.com>
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Robert Collins <rcollins@nvidia.com>