clk: ux500: add range to usleep_range

Providing a range for usleep_range() allows the hrtimer subsystem to
coalesce timers - the delay is runtime configurable so a factor 2
is taken to provide the range. With the expected range for
enable_delay_us being milliseconds, the range should lie in the 250us
range which is sufficient for hrtimer optimization.

Signed-off-by: Nicholas Mc Guire <hofrat@opentech.at>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>