reset: Add renesas,rst DT bindings

Add DT bindings for the Renesas R-Car Reset Controller (R-Car Gen1
RESET/WDT and R-Car Gen2/Gen3 and RZ/G RST).

As the features provided by the hardware module differ a lot across the
various SoC families and members, only SoC-specific compatible values
are defined.

For now we use the RST only for providing access to the state of the
mode pins, which is needed by the clock driver.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Magnus Damm <damm+renesas@opensource.se>
Acked-by: Dirk Behme <dirk.behme@de.bosch.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>