thunderbolt: Move struct tb_cap_any to tb_regs.h

This structure will be needed by the debugfs implementation so make it
available outside of cap.c.

While there add kernel-doc comments to the structure.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>