docs: watchdog: mlx-wdt: Add description of new watchdog type 3

Add documentation with details of new type of Mellanox watchdog driver.

Signed-off-by: Michael Shych <michaelsh@mellanox.com>
Reviewed-by: Vadim Pasternak <vadimp@mellanox.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20200504141427.17685-5-michaelsh@mellanox.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>

Documentation/watchdog/mlx-wdt.rst

@@ -24,10 +24,19 @@ Type 2:
   Maximum timeout is 255 sec.
   Get time-left is supported.
 
+Type 3:
+  Same as Type 2 with extended maximum timeout period.
+  Maximum timeout is 65535 sec.
+
 Type 1 HW watchdog implementation exist in old systems and
 all new systems have type 2 HW watchdog.
 Two types of HW implementation have also different register map.
 
+Type 3 HW watchdog implementation can exist on all Mellanox systems
+with new programmer logic device.
+It's differentiated by WD capability bit.
+Old systems still have only one main watchdog.
+
 Mellanox system can have 2 watchdogs: main and auxiliary.
 Main and auxiliary watchdog devices can be enabled together
 on the same system.
@@ -54,3 +63,4 @@ The driver checks during initialization if the previous system reset
 was done by the watchdog. If yes, it makes a notification about this event.
 
 Access to HW registers is performed through a generic regmap interface.
+Programmable logic device registers have little-endian order.