dt-bindings: power: reset: Unrequire regmap property in syscon-reboot node

Since normally syscon-reboot block is supposed to be a part of a system
controller, lets mark the regmap property as deprecated and recommend the
syscon-reboot node to be a sub-node of SYSCON.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>