MAINTAINERS: Add phy-stih407-usb.c file to ARCH/STI architecture

This patch adds the new phy-stih407-usb.c usb phy 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>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>