ARM: davinci: da850: do not specify clock_frequency for UART DT node

DT kernel on da850-evm comes up with garbled UART logs. This is because
of mismatch in actual module clock rate and rate specified(clock-frequency)
in DT blob. kernel should not assume or depend on bootloaders clock
configuration, instead let it find the clock rate at runtime.

Issue discussed here before arriving on this implementation:
"ARM: davinci: da850 evm: update clock rate for UART 1/2 DT nodes"
https://patchwork.kernel.org/patch/2162271/

Signed-off-by: Manjunathappa, Prakash <prakash.pm@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>