[POWERPC] iseries: Make ItExtVpdPanel private to iSeries

No one outside platforms/iseries needs ItExtVpdPanel anymore, so move
it in there. It used to be needed by lparcfg, and so was exported, but
isn't needed anymore, so unexport it.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>