bus/auxiliary: introduce auxiliary bus

bus/auxiliary: introduce auxiliary bus

Auxiliary bus [1] provides a way to split function into child-devices
representing sub-domains of functionality. Each auxiliary device
represents a part of its parent functionality.

Auxiliary device is identified by unique device name, sysfs path:
  /sys/bus/auxiliary/devices/<name>

Devargs legacy syntax of auxiliary device:
  -a auxiliary:<name>[,args...]
Devargs generic syntax of auxiliary device:
  -a bus=auxiliary,name=<name>/class=<class>/driver=<driver>[,args...]

[1] kernel auxiliary bus document:
https://www.kernel.org/doc/html/latest/driver-api/auxiliary_bus.html

Signed-off-by: Xueming Li <xuemingl@nvidia.com>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: Thomas Monjalon <thomas@monjalon.net>