lisa._assets.kmodules.lisa: Create VFS for lisa module

In order to configure lisa features and their parameters at
runtime, add support for a VFS in the lisa module.

The lisa VFS can be mounted as:
$ mount -t lisa none /sys/fs/lisa

It contains the following files/folders:
lisa_fs
\-activate                  # Activate this config
\-available_features        # List of available features
\-event__lisa__perf_counter # (E.g.) Dir to configure perf_counter feature
  \-generic_counters        # Generic counter to track
  \-pmu_raw_counters        # PMU raw counters to track
\-configs                   # Allow multiple configurations
  \-custom_conf_0           # Custom configuration ()
    \- ...                  # Similar files as in lisa_fs/
\-set_features              # Features to enable

A feature can have various parameters. For instance, the
'event__lisa__perf_counter' feature has two parameters that can
be configured: 'generic_counters', 'pmu_raw_counters'.

Multiple configurations can co-exist. The 'main' configuration is shown
above, and other custom configurations can be created in the 'configs'
directory.
Upon activation, the configuration is provided to all features. Each
feature is then responsible of merging/aggregating the configurations.

Original-author: Beata Michalska <beata.michalska@arm.com>
Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>