powerpc/mpc85xx: set clock-frequency for T4/B4 clockgen node

For T4/B4, the clockgen node compatible string is updated to version 2.
Add clock-frequency setting for this new version.

Signed-off-by: Tang Yuantian <Yuantian.Tang@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>