With scmi_mbox_chan_setup enabled to identify and setup both Tx and Rx,
let's consolidate setting up of both the channels under the function
scmi_mbox_txrx_setup.

Since some platforms may opt not to support notifications or delayed
response, they may not need support for Rx. Hence Rx is optional and
failure of setting one up is not considered fatal.
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>