dt-bindings: mfd: maxim,max77693: Convert to dtschema

Convert the MFD part of Maxim MAX77693 MUIC to DT schema format.  The
example DTS was copied from existing DTS (exynos4412-midas.dtsi), so
keep the license as GPL-2.0-only.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>