mips/vcth: Use generic 16550 uart driver

As the common code also handles baudrate switching, which the board
specific vct.c driver did not support, this is one of the rare
occassions where deleting code actually adds a feature :)

Signed-off-by: Detlev Zundel <dzu@denx.de>
Acked-by: Stefan Roese <sr@denx.de>
Acked-by: Shinya Kuribayashi <shinya.kuribayashi@necel.com>