binman: btool: Add Xilinx Bootgen btool

Add the Xilinx Bootgen as bintool. Xilinx Bootgen is used to create
bootable SPL (FSBL in Xilinx terms) images for Zynq/ZynqMP devices. The
btool creates a signed version of the SPL. Additionally to signing the
key source for the decryption engine can be passend to the boot image.

Signed-off-by: Lukas Funke <lukas.funke@weidmueller.com>
diff --git a/tools/binman/bintools.rst b/tools/binman/bintools.rst
index 20ee243..1336f4d 100644
--- a/tools/binman/bintools.rst
+++ b/tools/binman/bintools.rst
@@ -208,7 +208,7 @@
 
 
 Bintool: bootgen: Sign ZynqMP FSBL image
----------------------------------------------
+----------------------------------------
 
 This bintool supports running `bootgen` in order to sign a SPL for ZynqMP
 devices.