firmware: arm_scpi: add alternative legacy structures, functions and macros

This patch adds support for the Legacy SCPI protocol that is available
in very early JUNO versions and shipped Amlogic ARMv8 based SoCs. Some
Rockchip SoC are also known to use this version of protocol with
extended vendor commands.

In order to support the legacy SCPI protocol variant, we need to add the
structures and macros definitions that varies against the final SCPI v1.0
specification.

We add the indirection table for legacy commands set so that it can
co-exist with the standard v1.0 command set. It also adds bitmap field
for channel selection since the legacy protocol mandates to send only
selected subset of the commands on the high priority channel.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
[sudeep.holla@arm.com: Updated the changelog]
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>