power: supply: add Qualcomm PMI8998 SMB2 Charger driver

Add a driver for the SMB2 charger block found in the Qualcomm PMI8998
and PM660.

This driver is capable of utilising Qualcomm's Automatic Power Source
Detection (APSD) BC1.2 implementation, as well as Automatic Input
Current Limiting (AICL) to configure the maximum input current
limit of DCP (wall) chargers.

Quick Charge is not currently supported.

Most devices using the smb2 charger have a secondary dedicated charger
chip which is used in parallel to enable faster charger without
overheating. However, not all do, as a result to ensure safety until
these are supported, the maximum current is limited to ~1A via the
FAST_CHARGE_CURRENT_CFG register.

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
Tested-by: Joel Selvaraj <joelselvaraj.oss@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>