syscon: update syscon_regmap_lookup_by_phandle

syscon: update syscon_regmap_lookup_by_phandle

Change the function syscon_regmap_lookup_by_phandle()
introduced by commit 6c3af1f24e4b ("syscon: dm: Add a
new method to get a regmap from DTS") to have
Linux-compatible syscon API.

Same modification than commit e151a1c288bd ("syscon: add
Linux-compatible syscon API") solves issue when the node
identified by the phandle has several compatibles and is
already bound to a dedicated driver.

See Linux commit bdb0066df96e ("mfd: syscon: Decouple syscon
interface from platform devices").

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>