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

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