mfd: ti-keystone-devctrl: Add bindings for device state control

The Keystone II devices have a set of registers that are used to control
the status of its peripherals. This node is intended to allow access to
this functionality.

Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>