powerpc/85xx: Update timer-frequency prop in ptp_timer node of device tree

Fix up the device tree property associated with the IEEE 1588 timer
source frequency.  Currently we only support the IEEE 1588 timer source
being the internal eTSEC system clock (for those SoCs with IEEE 1588
support).  The eTSEC clock is ccb_clk/2.

Signed-off-by: Bhaskar Upadhaya <Bhaskar.Upadhaya@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>