gpio: stmpe: Add DT support for stmpe gpio This patch allows the STMPE GPIO driver to be successfully probed and initialised when Device Tree support is enabled. Bindings are mentioned in Documentation too. Acked-by: Lee Jones <lee.jones@linaro.org> Signed-off-by: Vipul Kumar Samar <vipulkumar.samar@st.com> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>