versal: Return ENVL_NOWHERE instead of ENVL_UNKNOWN

The system fails to boot without any environment location, so return
ENVL_NOWHERE when there's nowhere to store the environment instead
of ENVL_UNKNOWN.

The same change was also done by commit 50918d0d ("xilinx: Return
ENVL_NOWHERE instead of ENVL_UNKNOWN") for zynq and zynqmp.

Signed-off-by: T Karthik Reddy <t.karthik.reddy@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/e4ed36408f10159677ed8060bfd5289f3e0691fa.1637752614.git.michal.simek@xilinx.com