dt-bindings: pwm-qti-lpg: Add qcom,num-lpg-channels property

Instead of parsing the LPG channel numbers from the register size
cell, add a property to specify the LPG channel numbers directly.

Change-Id: If5fbaca2017edea314827aad47fef8fc324e8663
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>