ARM: at91/dt: binding: add precision to AIC documentation

In response to the "undocumented compatible strings" message, here is a
patch which is adding the precision of two "chips" that should be used for
the "atmel,<chip>-aic" compatibility string.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>