mfd: cros_ec: Instantiate properly CrOS FP MCU device

Support Fingerprint MCU as a special of CrOS EC devices. The current FP
MCU uses the same EC SPI protocol v3 as other CrOS EC devices on a SPI
bus.

When a MCU has fingerprint support (aka EC_FEATURE_FINGERPRINT), it is
instantiated as a special CrOS EC device with device name 'cros_fp'. So
regardless of the probing order between the actual cros_ec and cros_fp,
the userspace and other kernel drivers should not confuse them.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>