Add serivce account requirement for tiflash pod while restore (#2651)

[ti-chi-bot]

This is an automated cherry-pick of #2651

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• v1.6 (TiDB Operator 1.6 versions)
• v1.5 (TiDB Operator 1.5 versions)
• v1.4 (TiDB Operator 1.4 versions)
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• v1.2 (TiDB Operator 1.2 versions)

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[Oreoxmt]

/approve

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