max8903: cleans up confusing relationship between dc_valid, dok and dcm.

The max8903_charger.h file indicated that dcm and dok were not optional
when dc_valid is set.

It makes sense to have dok as a compulsory pin when dc_valid is given.
However dcm can be optionally wired to a fixed level especially when the
circuit is configured for dc power exclusively.

The previous implementation already allowed for this somewhat, however no
error was given if dok wasn't given whilst dc_valid was.

The new implementation enforces dok presence when dc_valid is given. Whilst
allowing dcm to be optional.

Signed-off-by: Chris Lapa <chris@lapa.com.au>

Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>