Move CONFIG_OF_LIBFDT to Kconfig

Move this option to Kconfig and tidy up existing boards.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/dra7xx_evm_uart3_defconfig b/configs/dra7xx_evm_uart3_defconfig
index 204483a..b67c02a 100644
--- a/configs/dra7xx_evm_uart3_defconfig
+++ b/configs/dra7xx_evm_uart3_defconfig
@@ -15,3 +15,4 @@
 CONFIG_SPI_FLASH_SPANSION=y
 CONFIG_SYS_NS16550=y
 CONFIG_TI_QSPI=y
+CONFIG_OF_LIBFDT=y