dt-bindings: arm: stm32: st,mlahb: Drop spurious "reg" property from example

"reg" is not documented nor used for st,mlahb, so drop it from the
example to fix the warning:

Documentation/devicetree/bindings/arm/stm32/st,mlahb.example.dtb: ahb@38000000: Unevaluated properties are not allowed ('reg' was unexpected)
        from schema $id: http://devicetree.org/schemas/arm/stm32/st,mlahb.yaml#

Since "reg" is dropped, the unit-address must be as well.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240523154208.2457864-1-robh@kernel.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>