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-zynq/Kconfig b/arch/arm/mach-zynq/Kconfig
index 79f831e..e543103 100644
--- a/arch/arm/mach-zynq/Kconfig
+++ b/arch/arm/mach-zynq/Kconfig
@@ -70,4 +70,7 @@
 config ZYNQ_SDHCI_MAX_FREQ
 	default 52000000
 
+source "board/xilinx/Kconfig"
+source "board/xilinx/zynq/Kconfig"
+
 endif