MAINTAINERS: update STi maintainer list

Remove myself as STi maintainer as I will no longer have access to
STi platforms, and remove Srini too, who now works on other
platforms.

Patrice will manage the pull requests.

Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Cc: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Acked-by: Patrice Chotard <patrice.chotard@st.com>
Acked-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>