dt-bindings: power: reset: Convert syscon-reboot-mode to DT schema

Modern device tree bindings are supposed to be created as YAML-files
in accordance with dt-schema. This commit replaces SYSCON reboot-mode
legacy bare text bindings with YAML file. As before the bindings file
states that the corresponding dts node is supposed to be compatible
"syscon-reboot-mode" device and necessarily have an offset property
to determine which register from the regmap is supposed to keep the
mode on reboot.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>