firmware: arm_scmi: Separate out tx buffer handling and prepare to add rx

Currently we pre-allocate transmit buffers only and use the first free
slot in that pre-allocated buffer for transmitting any new message that
are generally originated from OS to the platform firmware.

Notifications or the delayed responses on the other hand are originated
from the platform firmware and consumes by the OS. It's better to have
separate and dedicated pre-allocated buffers to handle the notifications.
We can still use the transmit buffers for the delayed responses.

In addition, let's prepare existing scmi_xfer_{get,put} for acquiring
and releasing a slot to identify the right(tx/rx) buffers.

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