MAINTAINERS: Add ahci_st.c to ARCH/STI architecture

This patch adds the ahci_st.c driver found on STMicroelectronics
stih41x consumer electronics SoC's into the STI arch section
of the maintainers file.

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: Arnd Bergmann <arnd@arndb.de>