pinctrl: abx500: Rejiggle platform data and DT initialisation

Platform Data is invariably populated for this driver, even when
booting with Device Tree. Thus the Device Tree probing code encased
within the first check for Platform Data will never executed, causing
the driver to fail when DT is enabled.

This patch fixes the aforementioned regression by rejigging the
probe() semantics to attempt to extract a platform ID from Device Tree
if one can not be sourced from platform data.

A pointer to GPIO platform data is always passed to the driver now, so
there's little point in checking for 'pdata' and executing the DT case if
it's not there. The difference between booting with DT and !DT is when
booting with DT, plat_id is not populated. Thus, in the DT case we have
to use a DT match table in order to find out which platform we're
executing on. So, we're changing the semantics here to only use the
match table if no plat_id is supplied though platform data.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
[edited commit message]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>