[kernel] Fix clock period vs frequency error

Calling `spinwait()` was hanging due to improper computation of the
clock rate because justin did a dumb at math. Also the period can be
greater than 1ns, so the clock's units were updated to microseconds.

src/kernel/clock.cpp

@@ -14,9 +14,9 @@ clock::clock(uint64_t rate, clock::source source_func, void *data) :
 }
 
 void
-clock::spinwait(uint64_t ns) const
+clock::spinwait(uint64_t us) const
 {
-	uint64_t when = m_source(m_data) + ns;
+	uint64_t when = m_source(m_data) + us;
 	while (value() < when);
 }