regmap: add regmap_read_poll_timeout() helper

Add the regmap_read_poll_timeout() macro based on the Linux implementation
to simplify register polling with configurable timeout and sleep.

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