ARM: zynq: Make SYS_VENDOR configurable

Add a string description for SYS_VENDOR to allow configuring boards from
other vendors than just "xilinx".

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/arm/mach-zynq/Kconfig

@@ -28,6 +28,7 @@ config SYS_BOARD
 	default "zynq"
 
 config SYS_VENDOR
+	string "Vendor name"
 	default "xilinx"
 
 config SYS_SOC