pc104: Add EXPERT dependency for PC104 Kconfig option

PC/104 device driver Kconfig options previously had an implicit EXPERT
dependency by way of an explicit ISA_BUS_API dependency. Now that these
driver Kconfig options select ISA_BUS_API rather than depend on it, the
PC104 Kconfig option should have an explicit EXPERT dependency.

The PC/104 form factor and bus architecture are common in embedded
and specialized systems, but uncommon in typical desktop setups. For
this reason, it is best to mask these devices and configurations via the
EXPERT Kconfig option because the majority of users will never need to
concern themselves with PC/104.

Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>