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