x86: fsp: Move struct fspinit_rtbuf definition to chipset header

All FSP spec v1.0 complaint FSP binary uses struct fspinit_rtbuf
as defined by the 1.0 spec, however there are FSPs that does not
follow 1.0 spec (possible due to that FSP predates the 1.0 spec),
and future FSP binary that is complaint to v1.1 spec defines an
optional paltform-specific runtime data in the struct fspinit_rtbuf.
Hence move the definition to chipset header.

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