MAINTAINERS: Add dwc3-st.c file to ARCH/STI architecture

This patch adds the new dwc3-st.c glue driver found on
STMicroelectronics stih407 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>
Signed-off-by: Felipe Balbi <balbi@ti.com>