watchdog: renesas_wdt: Add R-Car Gen2 support

Due to commits:
* "ARM: shmobile: Add watchdog support",
* "ARM: shmobile: rcar-gen2: Add watchdog support", and
* "soc: renesas: rcar-rst: Enable watchdog as reset trigger for Gen2",
we now have everything we needed for the watchdog to work on Gen2 and
RZ/G1.

However, on early revisions of some R-Car Gen2 SoCs, and depending on SMP
configuration, the system may fail to restart on watchdog time-out, and
lock up instead.

Specifically:
  - On R-Car H2 ES1.0 and M2-W ES1.0, watchdog restart fails unless
    only the first CPU core is in use (using e.g. the "maxcpus=1" kernel
    commandline option).
  - On R-Car V2H ES1.1, watchdog restart fails unless SMP is disabled
    completely (using CONFIG_SMP=n during build configuration, or using
    the "nosmp" or "maxcpus=0" kernel commandline options).

This commit adds "renesas,rcar-gen2-wdt" as compatible string for R-Car
Gen2 and RZ/G1, but also prevents the system from using the watchdog
driver in cases where the system would fail to restart by blacklisting
the affected SoCs, using the minimum known working revisions (ES2.0 on R-Car
H2, and ES3.0 on M2-W), and taking the actual SMP software configuration
into account.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Ramesh Shanmugasundaram <ramesh.shanmugasundaram@bp.renesas.com>
[Geert: blacklisting logic]
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>