power: pm8xxx-ccadc: Add pm8xxx ccadc subdevice

Until now only the BMS system was using the ccadc so there was
no need to create a separate ccadc driver.

However we can run in a configuration with BMS disabled
and clients won't be able to read battery current via ccadc.

Separate the ccadc from the bms, this change in is preparation
to add a ccadc api to read the battery current.

Change-Id: Ib96b146d91d01d196df9291eb23432cd430db4d0
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>
[sboyd: Take only 8921-core parts]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>