scripts/gdb: add helpers to find and list devices

Add helper commands and functions for finding pointers to struct device
by enumerating linux device bus/class infrastructure.  This can be used
to fetch subsystem and driver-specific structs:

  (gdb) p *$container_of($lx_device_find_by_class_name("net", "eth0"), "struct net_device", "dev")
  (gdb) p *$container_of($lx_device_find_by_bus_name("i2c", "0-004b"), "struct i2c_client", "dev")
  (gdb) p *(struct imx_port*)$lx_device_find_by_class_name("tty", "ttymxc1")->parent->driver_data

Several generic "lx-device-list" functions are included to enumerate
devices by bus and class:

  (gdb) lx-device-list-bus usb
  (gdb) lx-device-list-class
  (gdb) lx-device-list-tree &platform_bus

Similar information is available in /sys but pointer values are
deliberately hidden.

Link: http://lkml.kernel.org/r/c948628041311cbf1b9b4cff3dda7d2073cb3eaa.1561492937.git.leonard.crestez@nxp.com


Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: Stephen Boyd <...