arm920t/at91: use new clock.c features

This patch enables the new clock features from arm920t/at91/clock.c. This
is an required step to get at91rm9200_usart replaced by atmel_usart driver.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Cc: Jens Scharsig <js_at_ng@scharsoft.de>
Cc: Eric Bénard <eric@eukrea.com>