power: regulator: denied disable on always-on regulator

Don't disable regulator which are tagged as "regulator-always-on" in DT.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jack Mitchell <jack@embed.me.uk>
Tested-by: Jack Mitchell <jack@embed.me.uk>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Richard Röjfors <richard@puffinpack.se>
Tested-by: Richard Röjfors <richard@puffinpack.se>
Reviewed-by: Felix Brack <fb@ltec.ch>
Tested-by: Felix Brack <fb@ltec.ch>