serial: powerpc: Implement CONFIG_SERIAL_MULTI into mpc8260 serial drivers

Implement support for CONFIG_SERIAL_MULTI into both SCC and SMC mpc8260
serial drivers. These drivers were so far only usable directly, but this
patch also adds support for the multi method. This allows using more than
one serial driver alongside the mpc8260 drivers. Also, add a weak
implementation of default_serial_console() returning this driver.

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>