defconfig: sdm670: move to sdm670 specific defconfig

Create new defconfig for sdm670 as target dependency
needs to be differentiated.

And also remove sdm670 configurations in sdm845 config.

Change-Id: Ieff320ce7b0285fc6555939f8dbb78754f241412
Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>