gpio/em: Add Device Tree support

Update the Emma Mobile GPIO driver to add DT support.

The patch simply adds a two-cell xlate function and
updates the probe code to allow configuration via DT
using the "ngpios" property plus OF id in the same
style as gpio-mvebu.c. The code is also adjusted to
use postcore_initcall() to force early setup.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>