serial: mpc512x: Properly define CONFIG_SYS_PSC3 in config files

serial: mpc512x: Properly define CONFIG_SYS_PSC3 in config files

Some of the boards using the mpc512x serial driver didn't properly
define which PSC console to use. This caused breakage when building
with CONFIG_SERIAL_MULTI enabled. Fix this by defining the default
PSC console.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Rini <trini@ti.com>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Stefan Roese <sr@denx.de>