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>