Migrate the i.MX8MQ EVK board to the Driver Model (DM) PMIC framework.
This replaces legacy PMIC handling with DM_PMIC and enables proper
device-model support during SPL.
As part of this transition, enable CONFIG_SPL_DM and the required
DM subsystems such as I2C, PINCTRL, MMC, GPIO, and regulators so the
SPL boot flow continues to work correctly.
Board-specific SPL code is simplified accordingly by removing legacy
I2C, USDHC, and PMIC handling, relying instead on device tree
descriptions and DM drivers.