firmware: arm_scmi: Add configurable polling mode for transports

SCMI communications along TX channels can optionally be provided of a
completion interrupt; when such interrupt is not available, command
transactions should rely on polling, where the SCMI core takes care to
repeatedly evaluate the transport-specific .poll_done() function, if
available, to determine if and when a request was fully completed or
timed out.

Such mechanism is already present and working on a single transfer base:
SCMI protocols can indeed enable hdr.poll_completion on specific commands
ahead of each transfer and cause that transaction to be handled with
polling.

Introduce a couple of flags to be able to enforce such polling behaviour
globally at will:

 - scmi_desc.force_polling: to statically switch the whole transport to
   polling mode.

 - scmi_chan_info.no_completion_irq: to switch a single channel dynamically
   to polling mode if, at runtime, is determined that no completion
   interrupt was available for such channel.

Link: https://lore.kernel.org/r/20211220195646.44498-2-cristian.marussi@arm.com


Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>