AT91: clock: peripheral clocks can have other parent than mck

While registering clock allow to set parent clock other
than mck. It is useful for clocks than can be seen as
child clock of a peripheral.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Andrew Victor <linux@maxim.org.za>