Silicon/Qemu/Sbsa: Specify correct amount of MCFG table entries

The number of entries specified in the typedef influences the length of
the whole structure, even though only one entry is filled.  This makes
some operating systems assume the second entry is also valid.  Reflect
reality by reducing the number of entries to a single one.

Signed-off-by: Patrick Wildt <patrick@blueri.se>
Acked-by: Ard Biesheuvel <ardb@kernel.org>