stm32:exti: Add EXTI_TRIGGER_NONE and support it in exti_set_trigger

[smavridis]

Allow resetting/clearing exti triggers.

A few notes:

• I appended the new EXTI_TRIGGER_NONE enum value, so as to not break anyone that might have used hard-coded values (i.e. 2 instead of EXTI_TRIGGER_BOTH).
• An alternate implementation could be adding a dedicated exti_clear_trigger call, but i felt it was better to just add the functionality to the existing exti_set_trigger.

Let me know what you think of this request, any changes you want and what i can do to merge it.

Thank you for the great work on OpenCM3.

[smavridis]

Updated to track master.

[UncleGrumpy]

Any chance of getting this merged? These changes would actually be quite helpful in a project I am working on.

[smavridis]

Updated to follow master.