ARM: ensure delay timer has sufficient accuracy for delays

We have recently had an example of someone wanting to use a 90kHz timer
for the software delay loop.

udelay() needs to have at least microsecond resolution to allow drivers
access to a delay mechanism with a reasonable chance of delaying the
period they requested within at least a 50% marging of error, especially
for small delays.

Discussion about the udelay() accuracy can be found at:
	https://lkml.org/lkml/2011/1/9/37

Reject timers which are unable to supply this level of resolution.
Acked-by: NNicolas Pitre <nico@linaro.org>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/lib/delay.c

@@ -83,6 +83,12 @@ void __init register_current_timer_delay(const struct delay_timer *timer)
 			       NSEC_PER_SEC, 3600);
 	res = cyc_to_ns(1ULL, new_mult, new_shift);
 
+	if (res > 1000) {
+		pr_err("Ignoring delay timer %ps, which has insufficient resolution of %lluns\n",
+			timer, res);
+		return;
+	}
+
 	if (!delay_calibrated && (!delay_res || (res < delay_res))) {
 		pr_info("Switching to timer-based delay loop, resolution %lluns\n", res);
 		delay_timer			= timer;