NFC: nfcsim: Add support for sysfs control entry

The idea is to have a way to control and/or modify the behavior of the
nfcsim virtual devices.

This patch creates a folder tree in the debug filesystem. The debugfs is
usually mounted into /sys/kernel/debug and the nfcsim entries are
located in DEBUGFS/nfcsim/nfcX/ where X is either 0 or 1 depending on
the device you want to address.

These folders are empty for now and control entries will be added by
upcoming commits.

Signed-off-by: Thierry Escande <thierry.escande@collabora.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>