B4860/B4420: Add PLL_NUM define for B4420/B4860 to use SerDes2 Refclks re-configuration

B4860 has two PLL per SerDes whereas B4420 has one PLL per SerDes,
add their defines in arch/powerpc/include/asm/config_mpc85xx.h

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