ARM: dts: bcm2835: Change firmware compatible from simple-bus to simple-mfd

ARM: dts: bcm2835: Change firmware compatible from simple-bus to simple-mfd

The current binding for the RPi firmware uses the simple-bus compatible as
a fallback to benefit from its automatic probing of child nodes.

However, simple-bus also comes with some constraints, like having the ranges,
our case.

Let's switch to simple-mfd that provides the same probing logic without
those constraints.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20200924082642.18144-1-maxime@cerno.tech
Signed-off-by: Rob Herring <robh@kernel.org>