pcie: shell: Add help and argument check

Use SHELL_CMD_ARG() for argument check and add help, looking like:

...

$ pcie -h
  pcie - PCI(e) device information
  Subcommands:
    ls  :List PCIE devices
         Usage: ls [bus:device:function] [dump]

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>