ARM: at91: clock: add PMC_PLLICPR init function

To avoid the duplicated code, add the PMC_PLLICPR init function.

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