power: pmic/fan53555: allow dm be omitted by SPL

Allow the dm driver be omitted by SPL.

Cc: Quentin Schulz <foss+u-boot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>