net: pcs: xpcs: Add Synopsys DW xPCS platform device driver

Synopsys DesignWare XPCS IP-core can be synthesized with the device CSRs
being accessible over the MCI or APB3 interface instead of the MDIO bus
(see the CSR_INTERFACE HDL parameter). Thus all the PCS registers can be
just memory mapped and be a subject of the standard MMIO operations of
course taking into account the peculiarities of the Clause C45 CSRs
mapping. From that perspective the DW XPCS devices would look as just
normal platform devices for the kernel.

On the other hand in order to have the DW XPCS devices handled by the
pcs-xpcs.c driver they need to be registered in the framework of the
MDIO-subsystem. So the suggested change is about providing a DW XPCS
platform device driver registering a virtual MDIO-bus with a single
MDIO-device representing the DW XPCS device.

DW XPCS platform device is supposed to be described by the respective
compatible string "snps,dw-xpcs" (or with the PMA-specific compatible
string), CSRs memory space and optional peripheral bus and reference clock
sources. Depending on the INDIRECT_ACCESS IP-core synthesize parameter the
memory-mapped reg-space can be represented as either directly or
indirectly mapped Clause 45 space. In the former case the particular
address is determined based on the MMD device and the registers offset (5
+ 16 bits all together) within the device reg-space. In the later case
there is only 8 lower address bits are utilized for the registers mapping
(255 CSRs). The upper bits are supposed to be written into the respective
viewport CSR in order to select the respective MMD sub-page.

Note, only the peripheral bus clock source is requested in the platform
device probe procedure. The core and pad clocks handling has been
implemented in the framework of the xpcs_create() method intentionally
since the clocks-related setups are supposed to be performed later, during
the DW XPCS main configuration procedures. (For instance they will be
required for the DW Gen5 10G PMA configuration.)

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>