powerpc/boot: drop planetcore_set_serial_speed

Drop planetcore_set_serial_speed() which had no users since its
inception in commit fec60470 ("[POWERPC] bootwrapper: Add PlanetCore
firmware support") in 2007.

Signed-off-by: Arseny Solokha <asolokha@kb.kras.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>