riscv timer: provide a function for cpu_ticks

This is cheaper than the default of tc_cpu_ticks().

MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D35462

sys/riscv/riscv/timer.c

@@ -219,6 +219,8 @@ riscv_timer_attach(device_t dev)
 	sc->et.et_priv = sc;
 	et_register(&sc->et);
 
+	set_cputicker(get_timecount, sc->clkfreq, false);
+
 	return (0);
 }