firmware: arm_scmi: Introduce protocol handle definitions

Add basic protocol handles definitions and private data helpers.

A protocol handle identifies a protocol instance initialized against a
specific handle, it embeds all the references to the core SCMI transfer
methods that will be needed by a protocol implementation to build and
send its own protocol specific messages using common core methods.

As such, in the interface, a protocol handle will be passed down from
the core to the protocol specific initialization callback at init time.

Anyways, at this point only definitions are introduced, all protocols
initialization code and SCMI drivers probing is still based on the old
interface, so no functional change.

Link: https://lore.kernel.org/r/20210316124903.35011-3-cristian.marussi@arm.com


Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>