debugfs: add helper function to create device related seq_file

This patch adds a helper function that simplifies adding a
so-called single_open sequence file for device drivers. The
calling device driver needs to provide a read function and
a device pointer. The field struct seq_file::private will
reference the device pointer upon call to the read function
so the driver can obtain his data from it and do its task
of providing the file content using seq_printf() calls and
alike. Using this helper function also gets rid of the need
to specify file operations per debugfs file.

Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>