Move ICS CLK chip frequency calculation code into a common board library

Move ICS CLK chip frequency calculation code into a common board library

We have several boards that use the same ICS307 CLK chip to drive the
System clock and DDR clock.  Move the code into a common location so we
share it.

Convert the P2020DS board as the first to use the new common ICS307
code.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Timur Tabi <timur@freescale.com>