serial: Remove CONFIG_SERIAL_MULTI from config files

Remove any notion of CONFIG_SERIAL_MULTI from board config files.
Since CONFIG_SERIAL_MULTI is now enabled by default, it is useless
to specify this config option in the board config files. Therefore
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>