serial: Remove CONFIG_SERIAL_MULTI from serial drivers

Remove the support for not-CONFIG_SERIAL_MULTI part from serial
port drivers and some board files. Since CONFIG_SERIAL_MULTI is
now enabled by default, that part is a dead code. Remove it.

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