watchdog: ts4800: add hardware dependency

The Technologic Systems TS-4800 is an i.MX515 board, so its drivers
are useless unless building a SOC_IMX51 kernel, except for build
testing purposes.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Damien Riegel <damien.riegel@savoirfairelinux.com>
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>