serial: mpc52xx: make printout for type more generic

The printout for the type should be just "5xxx", so 512x users won't
wonder why they have a mpc52xx-type UART.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>