mfd: ene-kb3930: Add driver for ENE KB3930 Embedded Controller

This driver provides access to the EC RAM of said embedded controller
attached to the I2C bus as well as optionally supporting its slightly weird
power-off/restart protocol.

A particular implementation of the EC firmware can be identified by a
model byte. If this driver identifies the Dell Ariel platform, it
registers the appropriate cells.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Lee Jones <lee.jones@linaro.org>