mfd: Remove MFD_CROS_EC_SPI depends on OF

The ChromeOS EC SPI transport driver has a dependency on OF because it
uses some OF helpers from the <linux/of.h> header. But there isn't a
need for an explicit dependency since the header has stub functions if
CONFIG_OF is not defined.

Also, MFD_CROS_EC_SPI already depends on MFD_CROS_EC which in turn has
a dependency on OF so in practice can't be selected without CONFIG_OF.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Lee Jones <lee.jones@linaro.org>