x86: coreboot: Document the memory map

Add information about memory usage when U-Boot is started from coreboot.
This is useful when debugging. Also, since coreboot takes a chunk of
memory in the middle of SDRAM for use by PCI devices, it can help avoid
overwriting this with a loaded kernel by accident.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>