ppc/pnv: add a LPC Controller model for POWER9

The LPC Controller on POWER9 is very similar to the one found on
POWER8 but accesses are now done via on MMIOs, without the XSCOM and
ECCB logic. The device tree is populated differently so we add a
specific POWER9 routine for the purpose.

SerIRQ routing is yet to be done.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20190307223548.20516-7-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>