gpio: xilinx: Convert driver to DM

This patch is enabling GPIO_DM support to have an option to use this
driver together with zynq gpio driver.
!DM part is kept there till Microblaze is cleanup which will be done
hopefully soon.

Just a note:
There is no reason to initialize uc-priv->name because it is completely
unused.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>