powerpc/85xx: add missing QMAN frequency calculation

When CONFIG_SYS_FSL_QORIQ_CHASSIS2 is not defined, QMAN frequency will not
be initialized, and QMAN will have a wrong frequency display.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>