Introduce .clang-format configuration file to U-Boot, providing
significant improvements over the existing scripts/Lindent approach for
C code formatting.

Benefits of clang-format over scripts/Lindent:

- More comprehensive formatting rules: While Lindent relies on the basic
  'indent' tool with limited options, clang-format provides extensive
  control over code formatting with 800+ configuration options

- Better handling of complex code structures: clang-format understands
  modern C constructs and handles nested structures, macros, and complex
  expressions more intelligently

- IDE and editor integration: Native support in major development
  environments (VS Code, Vim, Emacs, etc.) enables real-time formatting

- Consistent results across environments: Eliminates variations between
  different versions of 'indent' tool and system configurations

- Active maintenance: clang-format is actively developed and updated,
  unlike the aging 'indent' tool

Alignment with Linux kernel practices:

Continues U-Boot alignment with Linux kernel development practices,
maintaining consistency between these closely related projects. The
Linux kernel adopted clang-format to modernize its code formatting
infrastructure and improve developer experience.

The .clang-format file is based on the Linux kernel configuration,
specifically copied from Linux kernel v6.16 tag, which itself builds
upon the initial introduction in commit d4ef8d3ff005c ("clang-format:
add configuration file").

Signed-off-by: Javier Tia <javier.tia@linaro.org>