Input: pinephone-keyboard - support the proxied I2C bus

The PinePhone keyboard case contains a battery managed by an integrated
power bank IC. The power bank IC communicates over I2C, and the keyboard
MCU firmware provides an interface to read and write its registers.
Let's use this interface to implement a SMBus adapter, so we can reuse
the driver for the power bank IC.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Link: https://lore.kernel.org/r/20220618165747.55709-4-samuel@sholland.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>