arm64: zynqmp: Do not include pm_cfg_obj.o when SPL is disabled

xilinx_zynqmp_mini configuration is throwing build error:
readlink: missing operand
Try 'readlink --help' for more information.

because CONFIG_ZYNQMP_SPL_PM_CFG_OBJ_FILE is not defined at all and
Makefile pass ifneq condition. Add SPL_BUILD dependency which is also
reflected in Kconfig.

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