imx8mp: phyboard-pollux-rdk: Convert to DM_PMIC

imx8mp: phyboard-pollux-rdk: Convert to DM_PMIC

Convert the board to use DM_PMIC instead of the legacy SPL I2C/PMIC
handling.

Changes include:
- Enable DM_PMIC, DM_PMIC_PCA9450, and SPL_DM_PMIC_PCA9450 in defconfig.
- Drop legacy SPL I2C and PMIC options.
- Remove manual I2C1 pad setup and legacy power_pca9450_init() usage.
- Use DM-based pmic_get() with the DT node "pmic@25".
- Update PMIC register programming to use struct udevice API.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Yannic Moog <y.moog@phytec.de>
Tested-by: Yannic Moog <y.moog@phytec.de>
Reviewed-by: Teresa Remmet <t.remmet@phytec.de>