x86: baytrail: Add documentation for FSP memory-down values

This patch adds the documentation for the memory-down parameters
of the Intel FSP. To configure a board without SPD DDR DIMM but
with onboard DDR chips. The values are taken from the coreboot
header:

	src/soc/intel/fsp_baytrail/chip.h

(git ID da1a70ea from 2016-01-16 as reference).

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