Support AT91CAP9 revC CPUs

The AT91CAP9 revC CPU has a few differences over the previous,
revB CPU which was distributed in small quantities only (revA was
an internal Atmel product only).

The revC silicon needs a special initialisation sequence to
switch from the internal (imprecise) RC oscillator to the
external 32k clock.

Signed-off-by: Stelian Pop <stelian@popies.net>
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>