[coreboot] amdk8_util (and amdfam10_util) fix and documentation

[Myles Watson]

This patch is mostly documenting the functionality, but it also fixes
a return value which wasn't added correctly when there is no resource.
 I've also added code in my tree to remove zero-sized resources, but
since it hasn't made me magically boot Windows yet... I guess it's not
worth risking breaking it for someone else.

Signed-off-by: Myles Watson

Thanks,
Myles