arm: imx7s-warp: Convert to DM PMIC

This patch converts the warp7 and warp7_bl33 board ports over to using the
DM PMIC model.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>