dpaa2: Avoid dpaa2_cmd race conditions

struct dpaa2_cmd is no longer malloc'ed, but can be allocated on stack
and initialized with DPAA2_CMD_INIT() on demand. Drivers stopped caching
their DPAA2 command objects (and associated tokens) in the software
contexts in order to avoid using them concurrently.

Reviewed by: bz
Approved by: bz (mentor)
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D39509

Add initial DPAA2 support

DPAA2 is a hardware-level networking architecture found in some NXP
SoCs which contain hardware blocks including Management Complex
(MC, a command interface to manipulate DPAA2 objects), Wire Rate I/O
processor (WRIOP, packets distribution, queuing, drop decisions),
Queues and Buffers Manager (QBMan, Rx/Tx queues control, Rx buffer
pools) and the others.

The Management Complex runs NXP-supplied firmware which provides DPAA2
objects as an abstraction layer over those blocks to simplify an
access to the underlying hardware. Each DPAA2 object has its own
driver (to perform an initialization at least) and will be visible
as a separate device in the device tree.

Two new drivers (dpaa2_mc and dpaa2_rc) act like firmware buses in
order to form a hierarchy of the DPAA2 devices:

acpiX (or simplebusX)
dpaa2_mcX
dpaa2_rcX
dpaa2_mcp0
...
dpaa2_mcpN
dpaa2_bpX
dpaa2_macX
dpaa2_io0
...
dpaa2_ioM
dpaa2_niX

dpaa2_mc is suppossed to be a root of the hierarchy, comes in ACPI
and FDT flavours and implements helper interfaces to allocate and
assign bus resources, MSI and "managed" DPAA2 devices (NXP treats some
of the objects as resources for the other DPAA2 objects to let them
function properly). Almost all of the DPAA2 objects are assigned to
the resource containers (dpaa2_rc) to implement isolation.

The initial implementation focuses on the DPAA2 network interface
to be operational. It is the most complex object in terms of
dependencies which uses I/O objects to transmit/receive packets.

Approved by: bz (mentor)
Tested by: manu, bz
MFC after: 3 months
Differential Revision: https://reviews.freebsd.org/D36638