Register timer for RV32

[ytakano]

1. Implement Timer trait.

awkernel/awkernel_lib/src/timer.rs

Lines 11 to 20 in 1679d1d

	pub trait Timer {
	/// Reset the timer interrupt.
	fn reset(&self, dur: Duration);
	/// Get IRQ#.
	fn irq_id(&self) -> u16;
	/// Disable the timer interrupt.
	fn disable(&self);
	}

2. Register the timer.

awkernel/awkernel_lib/src/timer.rs

Line 22 in 1679d1d

pub fn register_timer(timer: Box<dyn Timer + Send + Sync>) {