xilinx: Do not call private env_get_location() when !ENV_IS_NOWHERE

Private function for finding out location of environment is not working
when ENV_IS_NOWHERE is disabled.
The reason is that current fallback is ENVL_UNKNOWN when
CONFIG_ENV_IS_NOWHERE is not enabled. The code could be updated like this

-               return ENVL_NOWHERE;
+               if (IS_ENABLED(CONFIG_ENV_IS_NOWHERE))
+                       return ENVL_NOWHERE;
+               return ENVL_UNKNOWN;

But then boot is still not working because ENVL_UNKNOWN has no driver and
env_init() returns -ENODEV. That's why it is better not to define board
specific env_get_location() because then weak env_get_location() is used
which is going over env_locations[] and returning locations which are
actually enabled in u-boot instance.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/830b9c93d30688cfa8a5cbb0388e79deb7731f29.1711107795.git.michal.simek@amd.com
diff --git a/board/xilinx/zynqmp/zynqmp.c b/board/xilinx/zynqmp/zynqmp.c
index ba49eb7..79daaf8 100644
--- a/board/xilinx/zynqmp/zynqmp.c
+++ b/board/xilinx/zynqmp/zynqmp.c
@@ -588,6 +588,7 @@
 	return bootseq;
 }
 
+#if defined(CONFIG_ENV_IS_NOWHERE)
 enum env_location env_get_location(enum env_operation op, int prio)
 {
 	u32 bootmode = zynqmp_get_bootmode();
@@ -621,6 +622,7 @@
 		return ENVL_NOWHERE;
 	}
 }
+#endif
 
 #if defined(CONFIG_SET_DFU_ALT_INFO)