configs: imx8mn_beacon: Enable SPL_DM_PMIC_BD71837

To properly operate the Nano with LPDDR4 at 1.6GHz, the
voltage needs to be adjusted before DDR is initialized.
Enable the PMIC in SPL to do this.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>