arm64: dts: imx8mm-evk: Add BD71847 PMIC

The BUCK2 regulator is used for cpufreq voltage control, otherwise
configuration is mostly static.

This uses the newly-implemented rohm,reset-snvs-powered property to
properly handle the SNVS state of imx8mm.

Between BD71837 and BD71847 the BUCK3/4 regulators were removed but
datasheet and board schematics kept the names for BUCK5/6/7/8. The
driver however renumbered 5/6/7/8 to 3/4/5/6. Use the names from DT
bindings and add comments to signal this.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Acked-By: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>