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-versal/Kconfig b/arch/arm/mach-versal/Kconfig
index e1d66e8..ebd2da3 100644
--- a/arch/arm/mach-versal/Kconfig
+++ b/arch/arm/mach-versal/Kconfig
@@ -62,4 +62,7 @@
 	  This option configures MMU with no DDR to avoid speculative
 	  access to DDR memory where DDR is not present.
 
+source "board/xilinx/Kconfig"
+source "board/xilinx/versal/Kconfig"
+
 endif