ppc/ppc4xx: Introduce a DCR device model

The Device Control Registers (DCR) of on-SoC devices are accessed by
software through the use of the mtdcr and mfdcr instructions. These
are converted in transactions on a side band bus, the DCR bus, which
connects the on-SoC devices to the CPU.

Ideally, we should model these accesses with a DCR namespace and DCR
memory regions but today the DCR handlers are installed in a DCR table
under the CPU. Instead, introduce a little device model wrapper to hold
a CPU link and handle registration of DCR handlers.

The DCR device inherits from SysBus because most of these devices also
have MMIO regions and/or IRQs. Being a SysBusDevice makes things easier
to install the device model in the overall SoC.

Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
[balaton: Explicit opaque parameter for dcr callbacks]
Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Message-Id: <9b21bdf55e0a728f093bad299e030d98f302ded0.1660746880.git.balaton@eik.bme.hu>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>

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