more work on descriptor building, almost there

also added the very early beginnings of an example to document how to
get speed

examples/lpc55s28/src/main.rs

@@ -0,0 +1,20 @@
+use lpc55_pac as pac;
+use usbd_uac2::{self, UsbSpeed, UsbSpeedProvider};
+
+struct Lpc55UsbSpeedProvider {}
+
+impl UsbSpeedProvider for Lpc55UsbSpeedProvider {
+    fn speed(&self) -> usbd_uac2::UsbSpeed {
+        let regs = unsafe { &*pac::USB1::ptr() };
+        match regs.devcmdstat.read().speed().bits() {
+            1 => UsbSpeed::Full,
+            2 => UsbSpeed::High,
+            3 => UsbSpeed::Super,
+            _ => panic!("Unknown USB speed"),
+        }
+    }
+}
+
+fn main() {
+    println!("Hello, world!");
+}