mfd: cros ec: spi: Depend on OF

The driver is not used on any non-DT platform, so it can depend on the
OF Kconfig symbol to make that explicit. This is in preparation of a
subsequent patch which parses some parameters from the device tree and
would otherwise have to conditionalize that code.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>