Add .gitattributes for line endings

When building U-Boot host tools for Windows from Microsoft Azure
Pipelines, we see tons of weird warnings and errors emitted from
every Kconfig files:

  Kconfig:6:warning: ignoring unsupported character ''
  Kconfig:6:warning: ignoring unsupported character ''
  Kconfig:8:warning: ignoring unsupported character ''
  Kconfig:9:warning: ignoring unsupported character ''
  Kconfig:10:warning: ignoring unsupported character ''
  Kconfig:10:warning: ignoring unsupported character ''
  Kconfig:13:warning: ignoring unsupported character ''
  arch/Kconfig:1:warning: ignoring unsupported character ''
  arch/Kconfig:2:warning: ignoring unsupported character ''
  arch/Kconfig:2:warning: ignoring unsupported character ''
  arch/Kconfig:4:warning: ignoring unsupported character ''
  ...

After several rounds of experiments, it turns out this is caused
by line endings. Historically, Linux and macOS used linefeed (LF)
characters while Windows used a carriage return plus a linefeed
(CRLF). When Azure Pipelines checks out the U-Boot repo, Git tries
to compensate for the difference by automatically making lines end
in CRLF in the working directory on Windows, which confuses the
Kconfig file parsing logic.

Fortunately Git provides a way for repos to tell Git not to do such
automatical line endings conversion via .gitattributes file below:

* text eol=lf

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>