mfd: max77686/802: Remove support for board files

The driver is used only on Exynos based boards with DTS support.
After removal of board file support from max77686 and max77802 regulator
drivers, the MFD driver can be converted to DTS-only version. This
simplifies a little the code:
1. No dead (unused) entries in platform_data structure.
2. More code removed.
3. Regulator driver does not depend on allocated memory
   from MFD driver.
4. It makes also easier extending the regulator driver.

Add to the max77686 MFD driver dependency on CONFIG_OF because without
DTS the regulator drivers (max77686 and max77802) won't bind.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Tested-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Lee Jones <lee.jones@linaro.org>