ArmPkg/BdsLib: Fixed cache cleaning in PreparePlatformHardware()

Because the D&I caches were clean before to be disabled, the cache
lines might have got dirty during the cache maintenance operations.
This fix disables D&I caches before to clean them. The performance
drops should be minimised as invalidating the I cache is only a
couple of instruction.

Signed-off-by: Eugene Cohen <eugene@hp.com>
Reviewed-by: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@13503 6f19259b-4bc3-4df7-8a09-765794883524