firmware: arm_scmi: add initial support for power protocol

The power protocol is intended for management of power states of various
power domains. The power domain management protocol provides commands to
describe the protocol version, discover the implementation specific
attributes, set and get the power state of a domain.

This patch adds support for the above mention features of the protocol.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
--
 drivers/firmware/arm_scmi/Makefile |   2 +-
 drivers/firmware/arm_scmi/power.c  | 242 +++++++++++++++++++++++++++++++++++++
 include/linux/scmi_protocol.h      |  28 +++++
 3 files changed, 271 insertions(+), 1 deletion(-)
 create mode 100644 drivers/firmware/arm_scmi/power.c