(timer): basic start

src/kernel/timer.c

@@ -0,0 +1,12 @@
+#include <stdint.h>
+#include <kernel/interrupts.h>
+#include <kernel/plic.h>
+#include <kernel/timer.h>
+
+void timer_handle_interrupt()
+{
+	static volatile uint64_t *mtime = (uint64_t *)CLINT_MTIME;
+	static volatile uint64_t *mtimecmp = (uint64_t *)CLINT_MTIMECMP(0);
+
+    *mtimecmp = *mtime + TIMER_FREQ;
+}