x86: coreboot: Document cbmem console struct

Coreboot changed a few years ago to include an overflow flag. Update the
structure to match this.

This comes from coreboot commit:

   6f5ead14b4 ("mb/google/nissa/var/joxer: Update eMMC DLL settings")

Note: There are several implementations of this in coreboot. I have chosen
to follow the one in src/lib/cbmem_console.c

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