ArmPkg: Rectify file modes

Problems have been encountered because some of the source files have
execute permission set.  This can cause git to report them as changed
when they are checked out onto a file system with inherited permissions.
This has been seen using Cygwin, MinGW and PowerShell Git.

This patch makes no change to source file content, and only aims to
correct the file modes/permissions.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Evan Lloyd <evan.lloyd@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19778 6f19259b-4bc3-4df7-8a09-765794883524