tty: move drivers/serial/ to drivers/tty/serial/

The serial drivers are really just tty drivers, so move them to
drivers/tty/ to make things a bit neater overall.

This is part of the tty/serial driver movement proceedure as proposed by
Arnd Bergmann and approved by everyone involved a number of months ago.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Rogier Wolff <R.E.Wolff@bitwizard.nl>
Cc: Michael H. Warfield <mhw@wittsend.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>