media: atmel: atmel-isc: split the clock code into separate source file

The atmel-isc-base is getting crowded. Split the clock functions into
atmel-isc-clk.c.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>