edk2-platforms: Add __pycache__ to .gitignore

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=2217



Python build is enabled for some platform code leading to .pyc files
created in __pycache__ directories. This change adds the __pycache__
directory to a .gitignore file to allow git to ignore Python cache files.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Signed-off-by: Michael Kubacki <michael.a.kubacki@intel.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>