Add support for PWM (taken from stm32h7xx-hal). (#29)

This commit requires a recent git version of stm32g4 to compile, as the
last released version specifies some wrong register sizes and types.

The changes to the code include:
- stm32g4xx-hal uses a different API for clock gating/reset.
- stm32g4 is missing many bit/field names, so the code often uses .bits()
  or .set_bit()/.clear_bit() instead.
- The advanced-control timers of the STM32G4 MCUs provide four
  complementary outputs.

Author: mgottschlag

src/lib.rs

@@ -73,7 +73,7 @@ pub mod gpio;
 pub mod i2c;
 pub mod opamp;
 pub mod prelude;
-// pub mod pwm;
+pub mod pwm;
 // pub mod qei;
 pub mod rcc;
 // pub mod rng;

src/prelude.rs

@@ -35,7 +35,7 @@ pub use crate::serial::SerialExt as _;
 pub use crate::spi::SpiExt as _;
 pub use crate::time::U32Ext as _;
 // pub use crate::timer::opm::OpmExt as _;
-// pub use crate::timer::pwm::PwmExt as _;
+pub use crate::pwm::PwmExt as _;
 // pub use crate::timer::qei::QeiExt as _;
 // pub use crate::timer::stopwatch::StopwatchExt as _;
 // pub use crate::timer::TimerExt as _;