Documentation: dt: reset: move socfpga-reset

Instead of having the documentation for the socfpga-reset controller in a
vendor specific directory, move it to the reset folder to all the other
reset drivers.
Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Dinh Nguyen <dinguyen@altera.com>