mainboard: Add ASRock H81M-HDS

Tested with GRUB 2.02 as a payload, booting Debian GNU/Linux 9.5 with
kernel 4.9.

This board works quite well under coreboot. A list of what works and
what doesn't can be found in the documentation part of this commit.

The file `data.vbt` matches the VBT in the latest stable version of the
vendor firmware (version 2.20).

Change-Id: I53483bb9fa335e86e85dfc487fef03fce4b85e2a
Signed-off-by: Tristan Corrick <tristan@corrick.kiwi>
Reviewed-on: https://review.coreboot.org/29390
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

Documentation/mainboard/index.md

@@ -6,6 +6,10 @@ This section contains documentation about coreboot on specific mainboards.
 
 - [P8H61-M LX](asus/p8h61-m_lx.md)
 
+## ASRock
+
+- [H81M-HDS](asrock/h81m-hds.md)
+
 ## Cavium
 
 - [CN81XX EVB SFF](cavium/cn8100_sff_evb.md)