test: regmap: add regmap_read_poll_timeout test

Add test to regmap_read_poll_timeout() helper to check the timeout works
properly but cannot test proper condition matching since read/write calls
are not executed in sandbox.

Tested-by: Jerome Brunet <jbrunet@baylibre.com>
Acked-by: Jagan Teki <jagan@openedev.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>