This refactors the :platform:clock and related modules to remove legacy code and simplify module partitioning and generalize Baudrate and Frequency specification with user-defined literals.

Structural changes (breaking):

• Renamed :platform:clock.cortex -> :platform:systick: Moved modm_clock.cpp to :platform:cortex-m module
• Moved modm::platform::ClockControl for STM32 to :platform:rcc module.
• modm:platform:clock now is a module that contains common code.
• moved flash latency data into modm-devices.

Refactorings (breaking):

• Renamed modm::platform::ClockControl -> modm::platform::Rcc for consistency.
• Renamed modm::cortex::SysTickTimer -> modm::platform::SysTickTimer for consistency.
• Refactored ClockControl::setFlashLatency(Hz, mV) -> ClockControl::setFlashLatency<Hz, mV>(): Allows latency table lookup in constexpr at compile-time.
• Added Rcc::updateCoreFrequency<Hz>(): Sets the internal frequency values for delay functions.

User-defined literals (breaking) added to the modm::literals namespace:

• Added _Hz, _kHz and _MHz literals. Removed and replaced usage of modm::clock::Frequency enum.
• Added _Bd, _kBd and _MBd literals. Removed and replaced usage of modm::Uart::Baudrate enum.
• Added _bps, _kbpsand_Mbps` literals. Not used yet.

All literals are simply casting to a dimensionless uint32_t of integer units (no fractional units possible) to be lightweight and backwards compatible with modm.
The limitation here is that since baudrates and frequencies are passed around as template arguments, the C++ limitations regarding template argument types applies, specifically float cannot be passed, as well as custom types cannot be used (Like a type-safe frequency_t type).

Todo:

• Move Flash Wait-States into modm-devices. See [stm32] Add flash latency tables modm-devices#18.
• Check if clock tree can at least be partially generated, requires data from modm-devices. No, this is difficult, see Add clock trees and query interface modm-devices#17. To be added later.
• Check if using a enum class frequency_t : uint32_t {}; is an option for providing strongly-typed template arguments. Unfortunately enum class operators are severely restricted (no implicit conversion obviously). No, this would break a lot of ::initialize<>() code without any benefits.
• Wikipedia suggests Bd as Baudrate symbol, however, I personally find _Baud, _kBaud and _MBaud more intuitive. Well, Bd is a SI unit, so that settles it.
• Wikipedia suggests bit/s as bitrate symbol, however _bits looks wrong, so _bps was chosen. This is fine, other libraries use this convention too (Bytes/s is Bps then).

@rleh @chris-durand