regmap: add iopoll-like polling macro for regmap_field

This patch adds a macro regmap_field_read_poll_timeout that works
similar to the readx_poll_timeout defined in linux/iopoll.h, except
that this can also return the error value returned by a failed
regmap_field_read.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Mark Brown <broonie@kernel.org>