ARM: ensure delay timer has sufficient accuracy for delays

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: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>