firmware: arm_scmi: Segregate tx channel handling and prepare to add rx

The transmit(Tx) channels are specified as the first entry and the
receive(Rx) channels are the second entry as per the device tree
bindings. Since we currently just support Tx, index 0 is hardcoded at
all required callsites.

In order to prepare for adding Rx support, let's remove those hardcoded
index and add boolean parameter to identify Tx/Rx channels when setting
them up.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>