ath79: bsap18x0: specify FIS directory location in dts

The redboot-fis parser has option to specify the location of FIS
directory, use that, instead of patching the parser to scan for it, and
specifying location in kernel config.

Tested-by: Brian Gonyer <bgonyer@gmail.com>
Signed-off-by: Tomasz Maciej Nowak <tmn505@gmail.com>

target/linux/ath79/dts/ar7161_adtran_bsap1880.dtsi

@@ -86,6 +86,7 @@
 			#address-cells = <1>;
 			#size-cells = <1>;
 			compatible = "redboot-fis";
+			fis-index-block = <0xfd>;
 		};
 	};
 };