ppc/pnv: Increase max firmware size

Builds enabling GCOV can be bigger than 4MB and the limit on FSP
systems is 16MB.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20201002091440.1349326-1-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>