ARM: imx8mm: verdin-imx8mm: Drop superfluous header

ARM: imx8mm: verdin-imx8mm: Drop superfluous header

The power/bd71837.h should no longer be included, since V1.1 SoM
uses only the PCA9450 PMIC and the BD71837 support was removed.
Drop the header too.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@denx.de>
Cc: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Cc: Max Krummenacher <max.krummenacher@toradex.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Acked-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>