refactor: move pl011/pl031 to be separate features

kernel/src/dev/rtc/mod.rs

@@ -2,6 +2,7 @@
 
 use crate::dev::Device;
 
+#[cfg(feature = "pl031")]
 pub mod pl031;
 
 // TODO define what RTC devices can do

kernel/src/dev/serial/mod.rs

@@ -3,6 +3,7 @@
 use crate::dev::Device;
 use error::Errno;
 
+#[cfg(feature = "pl011")]
 pub mod pl011;
 
 /// Generic interface for serial devices