iio: pc104: Mask PC/104 drivers via the PC104 Kconfig option

PC/104 drivers should be hidden on machines which do not support PC/104
devices. This patch adds the PC104 Kconfig option as a dependency for
the relevant PC/104 device driver Kconfig options.

Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>