iio: st_accel: Support sensor i2c probe using acpi

Add support to probe st_accel sensors on i2c bus using ACPI. Compatible
strings are not avaialable on ACPI based systems.

Signed-off-by: Shrirang Bagul <shrirang.bagul@canonical.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>