conf.py: cpp_id_attributes = ["__sparse_cache"]

[marc-hb]

With a recent enough sphinx-build --version, this fixes the gazillion of warnings recently added:

/home/runner/work/sof-docs/sof-docs/api/component-api.rst:21: WARNING: Error when parsing function declaration.
If the function has no return type:
  Error in declarator or parameters-and-qualifiers
  Invalid C++ declaration: Expected identifier in nested name, got keyword: void [error at 18]
    static inline void comp_underrun (struct comp_dev *dev, struct comp_buffer __sparse_cache *source, uint32_t copy_bytes)
    ------------------^
If the function has a return type:
  Error in declarator or parameters-and-qualifiers
  If pointer to member declarator:
    Invalid C++ declaration: Expected '::' in pointer to member (function). [error at 33]
      static inline void comp_underrun (struct comp_dev *dev, struct comp_buffer __sparse_cache *source, uint32_t copy_bytes)
      ---------------------------------^
  If declarator-id:
    Invalid C++ declaration: Expecting "," or ")" in parameters-and-qualifiers, got "*". [error at 90]
      static inline void comp_underrun (struct comp_dev *dev, struct comp_buffer __sparse_cache *source, uint32_t copy_bytes)
      ------------------------------------------------------------------------------------------^

[marc-hb]

Small, easy to revert and fixing a gazillion of warnings: abusing my super power and merging this.