drivers: max77693: Move state container to common header

This prepares for merging some of the drivers between max77693 and
max77843 so the child MFD driver can be attached to any parent MFD main
driver.

Move the state container to common header file. Additionally add
consistent 'i2c' prefixes to its members (of 'struct i2c_client' type).

Signed-off-by: Krzysztof Kozlowski <k.kozlowski.k@gmail.com>
Acked-by: Sebastian Reichel <sre@kernel.org>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>