Documentation: DT: MIPS: lantiq: Add docs for the RCU bindings

Documentation: DT: MIPS: lantiq: Add docs for the RCU bindings

This adds the initial documentation for the RCU module (a MFD device
which provides USB PHYs, reset controllers and more).

The RCU register range is used for multiple purposes. Mostly one device
uses one or multiple register exclusively, but for some registers some
bits are for one driver and some other bits are for a different driver.
With this patch all accesses to the RCU registers will go through
syscon.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Rob Herring <robh@kernel.org>
Cc: john@phrozen.org
Cc: andy.shevchenko@gmail.com
Cc: p.zabel@pengutronix.de
Cc: kishon@ti.com
Cc: mark.rutland@arm.com
Cc: linux-mips@linux-mips.org
Cc: linux-mtd@lists.infradead.org
Cc: linux-watchdog@vger.kernel.org
Cc: devicetree@vger.kernel.org
Cc: linux-spi@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/17121/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>