ARM: at91: clock: add a new file to handle clock

ARM: at91: clock: add a new file to handle clock

To reduce the duplicated code, add a new file to accommodate
the peripheral's and system's clock handle code, shared with
the SoCs with different ARM core.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Tested-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>