power: reset: syscon-reboot: Add parental syscon support

Since normally syscon-reboot block is supposed to be a part of a system
controller, lets look for the syscon regmap in a parental DT node if
regmap property isn't specified. DT binding from now considers the regmap
property as deprecated.

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