ARM: zynq: Simplify zynq configuration

Extending Kconfig for adding new platform is a lot of work
for nothing. Setting SYS_CONFIG_NAME directly in Kconfig and
remove all dependencies on TARGET_ZYNQ_* options including SPL.
As a side-effect it also remove custom init folder for ps7_init_gpl.*
files. Folder is chosen based on device-tree file.

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