at91: switch to CLKDEV_LOOKUP

we do not change the clock naming convention so does not need to switch
the AVR32 yet

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