ARM: zynqmp: Make SYS_VENDOR configurable

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

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/arch/arm/cpu/armv8/zynqmp/Kconfig b/arch/arm/cpu/armv8/zynqmp/Kconfig
index e175e6e..499e1dd 100644
--- a/arch/arm/cpu/armv8/zynqmp/Kconfig
+++ b/arch/arm/cpu/armv8/zynqmp/Kconfig
@@ -28,6 +28,7 @@
 	default "zynqmp"
 
 config SYS_VENDOR
+	string "Vendor name"
 	default "xilinx"
 
 config SYS_SOC