eth: mpnic: add scaffolding for Meta Platforms NIC

MPNIC is the next network interface controller designed by Meta. Create
a bare PCI driver for it. Subsequent changes will flesh it out.

Registers live in BAR0 and are addressed as an array of 32 bit words,
which is why the CSR indices are a quarter of the byte offset. Most of
the register file is only accessible as 64 bit, so that is the only
accessor added here.

If a read returns all ones, a second read to an unused register with
reset value 0 is performed to determine if the device is gone. If the
device is gone, the mapped BAR0 pointer is cleared, so no further
accesses reach the bus.

For now, if mpnic_mmio_err() clears mpd->uc_addr0, nothing will
reassign it. The user will have to unbind and rebind the driver. Future
series will reset uc_addr0 from pci error handlers.

In addition to being used as a 0 value reference CSR, MPNIC_BDQ_SPARE is
also used to perform write flushes, because it can be read without
producing side effects.

Signed-off-by: Daniel Zahka <daniel.zahka@gmail.com>
Link: https://patch.msgid.link/20260924-linux-mpnic-v2-1-4badc9b58b9e@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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