This adds syscon_regmap_lookup_by_phandle_optional() function to get an
optional regmap.

It behaves the same as syscon_regmap_lookup_by_phandle() except where
there is no regmap phandle. In this case, instead of returning -ENODEV,
the function returns NULL. This makes error checking simpler when the
regmap phandle is optional.
Suggested-by: NNicolas Boichat <drinkcat@chromium.org>
Signed-off-by: NEnric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: NMatthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NLee Jones <lee.jones@linaro.org>