docs: Move brcm,bcm21664-resetmgr.txt

Sebastian pointed out that .../bindings/reset is usually used for
peripheral reset controllers, whereas .../bindings/power/reset is
used for Board/System reset controllers.
Reported-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Chris Brand <chris.brand@broadcom.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>