sfc: Specify that the VFDI status page has page alignment and size

This requirement was meant to be implied in the name 'status page'.
One out-of-tree VF driver allocates a buffer using the structure size
and not a full page - hence the current odd specification - but in
practice that allocation will be padded and aligned to at least 4KB.
Therefore, we can specify this and have the option to extend the
structure up to 4KB without worrying about VF drivers using odd-shaped
buffers.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>