[POWERPC] 8xx: Add clock-frequency to adder875 and mpc885ads dts

cpm_uart_core has a dependency on fsl,cpm-brg/clock-frequency, this
means that a .dts that uses the cpm uart driver needs to supply a
clock-frequency entry for get_brgfreq to return a meaningful number.

Signed-off-by: Bryan O'Donoghue <bodonoghue@codehermit.ie>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>