checkpatch: Check to make sure DT property names are lowercase

Property names in DT files should be lowercase. Reports an error
if a property name that has uppercase characters in it is detected.
See following imaginary example:

qcom,venus@aae0000 {
	...
	ClockNames = "core_clk", "iface_clk", "bus_clk";
	...
};

"ClockNames" here is an invalid property name as it contains uppercase
characters.

Change-Id: I8f05e0daa84bc18b95e307c3733d4700ae7fe1b0
Signed-off-by: Prathyush Katukojwala <pkatukoj@codeaurora.org>