RccBus should be public

[bgamari]

Currently stm32f1xx_hal::rcc::RccBus is private, despite the associated Bus type being used in some public interfaces (e.g. GetBusFreq). This means it is impossible (AFAICT) to use these public interfaces while abstracting over peripherals. For instance, you might want to write:

<Timer::Bus as GetBusFreq>::get_timer_frequency(clocks)

while abstracting over Timer.

[TheZoq2]

There is some discussion on why this is the case in #161. I'll leave this open because there was some mention of making a public api around this in that PR