thermal: adc_tm: Add support to register channels without trips

Sometimes it is required to expose ADC channels to thermal
framework when there is no ADC_TM peripheral available.
Add support to add channels under ADC_TM to register with
thermal framework without registering interrupts.

Change-Id: I10d4aed2a8837eced36476dc5a0e60ed767287b6
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>