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/configs/xilinx_zynqmp_mini_qspi_defconfig b/configs/xilinx_zynqmp_mini_qspi_defconfig
index 706e798..23a690a 100644
--- a/configs/xilinx_zynqmp_mini_qspi_defconfig
+++ b/configs/xilinx_zynqmp_mini_qspi_defconfig
@@ -8,8 +8,8 @@
 CONFIG_SPL=y
 CONFIG_SYS_MEM_RSVD_FOR_MMU=y
 CONFIG_ZYNQMP_NO_DDR=y
-# CONFIG_PSCI_RESET is not set
 # CONFIG_CMD_ZYNQMP is not set
+# CONFIG_PSCI_RESET is not set
 CONFIG_DEFAULT_DEVICE_TREE="zynqmp-mini-qspi"
 # CONFIG_EXPERT is not set
 # CONFIG_LEGACY_IMAGE_FORMAT is not set