test: dm: regmap: Fix the long test delay

At present one of the regmap tests takes 5 seconds to run since it waits
for a timeout. This should be handled using sandbox_timer_add_offset()
which advances time for test purposes.

This requires a little change to make the regmap_read_poll_timeout()
testable.

Update the macro and the test.

Fixes: ebe3497c ("test: regmap: add regmap_read_poll_timeout test")

Signed-off-by: Simon Glass <sjg@chromium.org>