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>

arch/arm/boot/dts/da850.dtsi

@@ -152,23 +152,20 @@
 		serial0: serial@1c42000 {
 			compatible = "ns16550a";
 			reg = <0x42000 0x100>;
-			clock-frequency = <150000000>;
 			reg-shift = <2>;
 			interrupts = <25>;
 			status = "disabled";
 		};
 		serial1: serial@1d0c000 {
 			compatible = "ns16550a";
 			reg = <0x10c000 0x100>;
-			clock-frequency = <150000000>;
 			reg-shift = <2>;
 			interrupts = <53>;
 			status = "disabled";
 		};
 		serial2: serial@1d0d000 {
 			compatible = "ns16550a";
 			reg = <0x10d000 0x100>;
-			clock-frequency = <150000000>;
 			reg-shift = <2>;
 			interrupts = <61>;
 			status = "disabled";