MAINTAINERS: extend mvebu SoC entry with pinctrl drivers

There was no entry for the mvebu pinctrl drivers. As they are tightly
linked to the SoCs and there is a lot of common code to support the
various pinctrl of each SoCs, then add a new entry for the mvebu
maintainers.

Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>