pinctrl/abx500: add AB8500 sub-driver

This adds a subdriver for the AB8500 pinctrl portions.
As the pin controller (also the ABx500 controllers) is an
inherent part of the SoC and will prevent boot if not
available, select this from the Ux500 SoC Kconfig.

Acked-by: Olof Johansson <olof@lixom.net>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>