Drive clock pin as an output insted of a weak input

[petrsimi]

While I was testing some Integrated circuits, I found out that some of them did not work when the "C" signal type was used to drive clock pin. The reason was that the HIGH level generated using INPUT_PULLUP mode was to weak and the IO did not detected it as a HIGH level. This pull request resoved it.

Was there any specific reason to drive the high level using internal pull up in the input mode?

[gazcbm]

@petrsimi Did you find adverse issues with this change in your testing?

[petrsimi]

No, I am using it since then without any problem.