mtd: ts5500: Add dependency

There is no point in displaying the TS5500-specific driver entries if
TS5500 board support itself isn't enabled.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>