x86: Add file names from Kconfig in descriptor/intel-me nodes in u-boot.dtsi

Since we now have the file names configurable via Kconfig for the flash
descriptor and intel-me files, add these from Kconfig in the corresponding
dts nodes.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>