B4860qds: Set SerDes2 refclk2 at to 156.25MHz for XFI to work

Change setting of SerDes2 refclk2 to have the default value as it is
coming on board that is 156.25MHz, for XFI to work.
Also change PLL_NUM variable to the one defined in config_mpc85xx.h
for B4860 and B4420.

Signed-off-by: Shaveta Leekha <shaveta@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>