serial: 8250: remove AR7 support

AR7 is going to be removed from the Kernel, so remove its type
definition from 8250 code. As with previous removals, I checked with
Debian Code Search that 'PORT_AR7' is not used in userspace.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>