powerpc/spufs: allow spufs files to specify sizes

Currently, spufs never specifies the i_size for the files in context
directories, so stat() always reports 0-byte files.

This change adds allows the spufs_dir_(nosched_)contents arrays to
specify a file size. This allows stat() to report correct file sizes,
and makes SEEK_END work.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>