wdatwd: Add support for ACPI WDAT based watchdog timer.

Simply said, WDAT is an abstraction for the real WDT hardware. For
instance, to add a newer generation WDT to ichwd(4), one must know the
detailed hardware registers, etc..

With WDAT, the necessary IO accesses to operate the WDT are comprehensively
described in it and no hardware knowledge is required.

With this driver, the WDT on Advantech ARK-1124C, Dell R210 and Dell R240 are
detected and operated flawlessly.

• While R210 is also supported by ichwd(4), others are not supported yet.

The unfortunate thing is that not all systems have WDAT defined.

Submitted by: t_uemura at macome.co.jp
Reviewed by: hrs
Differential Revision: https://reviews.freebsd.org/D37493