iopoll: Tweak min sleep time used with usleep_range()

Users of the iopoll macros may need to poll addresses that take
a long time to update. In such cases, the hardcoded value of 1 us
passed as the minimum delay to usleep_range() may result in
reads that occur more often than necessary.

Use (sleep_us/4) as the min delay instead of 1 us to resolve this
is a scalable way.

Change-Id: I8f679a7ecdb9ea2666c78aabca3c8ba806e0ead3
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>