soc: renesas: Add R-Car RST driver

Add a driver for the Renesas R-Car Gen1 RESET/WDT and R-Car Gen2/Gen3
and RZ/G RST module.

For now this driver just provides an API to obtain the state of the mode
pins, as latched at reset time.  As this is typically called from the
probe function of a clock driver, which can run much earlier than any
initcall, calling rcar_rst_read_mode_pins() just forces an early
initialization of the driver.

Despite the current simple and almost identical handling for all
supported SoCs, the driver matches against SoC-specific compatible
values, as the features provided by the hardware module differ a lot
across the various SoC families and members.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Dirk Behme <dirk.behme@de.bosch.com>