spi: mpc512x-psc: adapt mpc5121-psc document to reality

The drivers support MPC5125 additionally to MPC5121, and there is an spi
mode that is also supported. Additionally some minor corrections are
done.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>