drm: xilinx: Kconfig: Remove depend on ARCH_ZYNQ

The Xilinx DRM is not ZYNQ specific anymore. Remove the dependency
from Kconfig.

Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>