firmware: arm_scpi: add command indirection to support legacy commands

Since the legacy SCPI and the SCPI v1.0 differ in the command values,
it's better to create some sort of command indirection in the driver
to avoid repeated version check at multiple places.

This patch adds the indirection command table to allow different values
of the command across SCPI versions.

[narmstrong@baylibre.com: added cmd check in scpi_send_message]
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>