Add new structures and functions to handle AT91 SoC detection.

[alexandre.belloni@free-electrons.com: reworked DBGU detection]
Signed-off-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: NBoris BREZILLON <boris.brezillon@free-electrons.com>
Signed-off-by: NNicolas Ferre <nicolas.ferre@atmel.com>