watchdog: Add i.MX7ULP watchdog support

The i.MX7ULP Watchdog Timer (WDOG) module is an independent timer
that is available for system use.
It provides a safety feature to ensure that software is executing
as planned and that the CPU is not stuck in an infinite loop or
executing unintended code. If the WDOG module is not serviced
(refreshed) within a certain period, it resets the MCU.

Add driver support for i.MX7ULP watchdog.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/1566999303-18795-2-git-send-email-Anson.Huang@nxp.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>