ARM64: zynqmp: Use zynqmp specific compatible string for gpio

The patch:
"gpio: Added support to Zynq Ultrascale+ MPSoC"
(sha1: bdf7a4ae)
added zynqmp specific features. This patch is switching the driver to
use the zynqmp compatible string.
Also enable the driver for ep108 platform.

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