x86: fsp: Add access to variable MRC data

x86: fsp: Add access to variable MRC data

With FSP2 the non-volatile storage used by the FSP to init memory can be
split into a fixed piece (determined at compile time) and a variable piece
(determined at run time). Add support for reading the latter.

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