editorconfig: Add C++ indentation style

This commit adds the indentation style for the C++ source and header
files.

Note that the C++ indentation style has been configured to match that
of the C files because the C++ source files we currently have in the
Zephyr repository follow the style we enforce for the C files.

In the future, it may be preferable to follow the indentation style
recommended by one of the common C++ style guides, such as the Google
C++ Style Guide which recommends 2 spaces for indentation.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>