ADC interface documentation

[rleh]

uint16_t readChannel(Channel channel)

According to docs.modm.io

typedef uint8_t 	Channel
// this type may also be an enum or enum class 

the call readChannel(1) should be legal, but: error: cannot convert 'int' to 'modm::platform::Adc1::Channel'.

We should update the documentation (by removing the fake typedef in architecture/).

cc @hshose