bcm283x: fix missing docs in aux uart

kernel/src/device/serial/bcm2835_aux_uart.rs

@@ -1,6 +1,4 @@
 //! Broadcom BCM2835 mini-UART driver
-// TODO
-#![allow(missing_docs)]
 
 use abi::{
     error::Error,
@@ -62,6 +60,7 @@ struct Inner {
     regs: IrqSafeSpinlock<DeviceMemoryIo<'static, Regs>>,
 }
 
+/// Broadcom 283x mini-UART driver
 pub struct Bcm2835AuxUart {
     base: PhysicalAddress,
     irq: FullIrq,