power: pmic/regulator allow dm be omitted by SPL

Allow the dm driver be omitted by SPL.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>