MAINTAINERS: Add sdhci-st file to ARCH/STI architecture

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Olof Johansson <olof@lixom.net>