usbfs: add support for the USBFS peripheral

Adds support for the USB full speed device peripheral.

Implements the `Device` function using the [usb-device](https://github.com/rust-embedded-community/usb-device) crate.

Creates a new `usbfs` module to allow for adding the `OTG` and `Host`
functions in later work.

Cargo.toml

@@ -16,6 +16,8 @@ void = { version = "1.0.2", default-features = false }
 cast = { version = "0.2.3", default-features = false }
 vcell = "0.1.2"
 embedded-dma = "0.1.2"
+critical-section = "1.1.0"
+usb-device = "0.2.9"
 
 [dependencies.gd32vf103-pac]
 version = "0.5.0"

src/lib.rs

@@ -28,4 +28,5 @@ pub mod signature;
 pub mod spi;
 pub mod time;
 pub mod timer;
+pub mod usbfs;
 pub mod watchdog;

src/usbfs.rs

@@ -0,0 +1,16 @@
+//! USB full-speed
+
+pub mod device;
+
+/// Settings for port speed.
+///
+/// In Host mode, either setting is acceptable.
+///
+/// In Device mode, port speed is fixed to full-speed.
+#[repr(u8)]
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum PortSpeed {
+    FullSpeedHost = 0b01,
+    LowSpeedHost = 0b10,
+    FullSpeedDevice = 0b11,
+}