sunxi: binman: Provide a default BL31 filename

Prior to commit 7f7f8aca8257 ("sunxi: Convert 64-bit boards to use
binman"), if the BL31 environment variable was not defined, the firmware
would be loaded from a file "bl31.bin" in the current directory. Restore
that behavior by providing that as the default filename in case no entry
arg is provided, which will be the case if the environment variable is
unset.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>