pinctrl: add abx500 pinctrl driver core

This adds the AB8500 core driver, which will be utilized by
the follow-on drivers for different ABx500 variants.
Sselect the driver from the DBX500_SOC, as this chip is
powering and clocking that SoC.

Cc: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>