mpc52xx_psc_spi: fix it for CONFIG_PPC_MERGE

Fix mpc5200 PSC SPI driver to actually work for CONFIG_PPC_MERGE
 - s/mpc52xx/mpc5200/, as this was changed in device tree some time ago
 - fix spi id detection

The move from 'ppc' to 'powerpc' didn't quite match the latest
kernel.org code.

Signed-off-by: Domen Puncer <domen.puncer@telargo.com>
Acked-by: Dragos Carp <dragos.carp@toptica.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>