Include kernel.h for GENMASK(), kstrtobool() and types.

Add forward declaration for struct adf_accel_dev. Remove unneeded
include.

This change doesn't introduce any function change.
Signed-off-by: Lucas Segarra Fernandez <lucas.segarra.fernandez@intel.com>
Reviewed-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>