HSI: Add common DT binding for HSI client devices

Implement and document generic DT bindings for HSI clients.

Signed-off-by: Sebastian Reichel <sre@kernel.org>
Reviewed-by: Pavel Machek <pavel@ucw.cz>
Tested-By: Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>

diff --cc Documentation/devicetree/bindings/hsi/client-devices.txt
Simple merge

diff --cc drivers/hsi/hsi.c
Simple merge

diff --cc include/linux/hsi/hsi.h
Simple merge