ARM: at91: use chipid device for soc detection

So far, the CIDR and EXID registers were in the DBGU interface. This device
has disappeared with the SAMA5D2 family. These registers are exposed
through a new device called chipid.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
[nicolas.ferre@atmel.com: remove useless warnings]
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
[arnd@arndb.de: suggest to use static functions to reduce scope]
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>