mfd / platform: cros_ec: Add sensor_count and make check_features public

Add a new function to return the number of MEMS sensors available in a
ChromeOS Embedded Controller. It uses MOTIONSENSE_CMD_DUMP if available
or a specific memory map ACPI registers to find out.

Also, make check_features public as it can be useful for other drivers
to know what the Embedded Controller supports.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>