[coreboot] [patch] DDR3 support of AMD Family 10

[Bao, Zheng]

DDR3 supporting is added.

More testing need to be done. But we don't have boards that cover
all the options.

Features that have been tested.
The socket AM3 and ASB2.
Single rank and double rank DIMM.

Features that need to be tested.
Socket C32 and G34, I don't know if it is DDR3.
Quad rank.
Registered DIMM, which is quite complicated and I am not confident
about.

The tilapia board is the demo that shows the way to use DDR3.
It is quite close to mahogany, except tilapia uses AM3.
For a new board, just specify the socket type in Kconfig the
devicetree.cb.

Signed-off-by: Zheng Bao