arm: Remove incorrect comment in arm_timer

The current comment says that the arm_timers are restricted to between
32 KHz and 1 MHz, but sp804 TRM does not specify those limits.

Signed-off-by: Mark Langsdorf <mark.langsdorf@calxeda.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
