watchdog: imx2_wdt: add pretimeout function support

The change adds watchdog pretimeout notification handling to imx2_wdt
driver, if device data contains information about a valid interrupt.

It is unlikely but still possible (e.g. through a software limitation)
that only a subset of watchdogs on SoC has interrupt lines, hence
functionally the devices from these two groups have different
capabilities, and this is reflected in different watchdog_info
structs assigned to the devices.

Signed-off-by: Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>