watchdog: s3c2410: Fix infinite interrupt in soft mode

In soft (no-reboot) mode, the driver self-pings watchdog upon expiration
of an interrupt.  However the interrupt itself was not cleared thus on
first hit, the system enters infinite interrupt handling loop.

On Odroid U3 (Exynos4412), when booted with s3c2410_wdt.soft_noboot=1
argument the console is flooded:
	# killall -9 watchdog
	[   60.523760] s3c2410-wdt 10060000.watchdog: watchdog timer expired (irq)
	[   60.536744] s3c2410-wdt 10060000.watchdog: watchdog timer expired (irq)

Fix this by writing something to the WTCLRINT register to clear the
interrupt.  The register WTCLRINT however appeared in S3C6410 so a new
watchdog quirk and flavor are needed.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>