pinctrl: mvebu: move to its own directory

Like the spear platform, the mvebu platform has multiple files: one
core file, and then one file per SoC family. More files will be added
later, as support for mach-orion5x and mach-mv78xx0 SoCs is added to
pinctrl-mvebu. For those reasons, having a separate subdirectory,
drivers/pinctrl/mvebu/ makes sense, and it had already been suggested
by Linus Wallej when the driver was originally submitted.

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