ARM: at91: add soc detection infrastructure

Add new structures and functions to handle AT91 SoC detection.

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