MAINTAINERS: Add phy-stih41x-usb.c to ARCH/STI architecture

This patch adds the new phy-sti41x-usb.c PHY 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>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>