serial: powerpc: Implement CONFIG_SERIAL_MULTI into amirix serial driver

Implement support for CONFIG_SERIAL_MULTI into amirix serial driver.
This driver was 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 amirix driver. 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>