watchdog: renesas-wdt: Add support for WDIOF_CARDRESET

This patch adds the WDIOF_CARDRESET support for the Renesas platform
watchdog, to know if the board reboot is due to a watchdog reset.

This is done via the WOVF bit (bit 4) of the RWTCSRA register, which
indicates if RWTCNT overflowed, triggering the reset in last boot.

Signed-off-by: Veeraiyan Chidambaram <veeraiyan.chidambaram@in.bosch.com>
[takeshi.kihara.df: changed to read the RWTCSRA register while clock is
 enabled]
Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>