There are few boards DTS using "hisi,rst-syscon" property -
undocumented "hisi" prefix.  The property will not be changed in DTS to
non-deprecated one, because of compatibility reasons.  Add deprecated
"hisi" prefix to silence DT schema warnings.

Cc: Wei Xu <xuwei5@hisilicon.com>
Cc: David Heidelberg <david@ixit.cz>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: David Heidelberg <david@ixit.cz>
Reviewed-by: Wei Xu <xuwei5@hisilicon.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220221082228.34407-1-krzysztof.kozlowski@canonical.com