xilinx: kconfig: Move sourcing of board Kconfig to mach folders

Do not source xilinx board Kconfig by other boards. These configs should be
available only when Xilinx platforms are selected.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/arch/arm/mach-zynqmp/Kconfig b/arch/arm/mach-zynqmp/Kconfig
index d82a737..f1301f6 100644
--- a/arch/arm/mach-zynqmp/Kconfig
+++ b/arch/arm/mach-zynqmp/Kconfig
@@ -171,4 +171,7 @@
 
 endchoice
 
+source "board/xilinx/Kconfig"
+source "board/xilinx/zynqmp/Kconfig"
+
 endif