ARM: STi: DT: Move reset controller constants into common location

By popular vote, the DT binding includes for reset controllers are located
in include/dt-bindings/reset/. Move the STi reset constants in there, too,
to avoid confusion.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Patrice Chotard <patrice.chotard@st.com>