powerpc/t4qds: use clock measurement for sysclk and ddr clock

Use QIXIS measurement registers to obtain sysclk and ddr clock. This
allows using non-standard clock speeds, set by directly writing to
clock chip or store the values in qixis clock data eeprom.

Signed-off-by: Ed Swarthout <Ed.Swarthout@freescale.com>
Signed-off-by: York Sun <yorksun@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>