regulator: 88pg86x: new i2c dual regulator chip

This chip is found on Google Chromecast and Valve Steam Link devices.
It provides two DC regulators with I2C voltage control, separate GPIO
enable pins and one sleep mode pin.

This driver does not expose GPIO functionality, but supports voltage
control in 1.0-2.2V range, based on I2C register information given in
Chromecast kernel driver by Jisheng Zhang.

Cc: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Signed-off-by: Alexander Monakov <amonakov@ispras.ru>
Signed-off-by: Mark Brown <broonie@kernel.org>