MAINTAINERS: pinctrl: Add git tree to Samsung pinctrl entry

Add a Git tree on @kernel.org for maintaining the Samsung pinctrl
drivers.  The tree will be maintained in a shared model between current
Samsung pinctrl maintainers.  Pull requests will be going to Linus
Walleij.

Also add the patchwork for linux-samsung-soc mailing list which will be
used for handling the patches.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Tomasz Figa <tomasz.figa@gmail.com>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>