arm64: dts: hisilicon: Use phandles for overriding nodes in hi6220

When overriding nodes, usage of phandles instead of full paths reduces
possible mistakes (e.g.  in duplicated unit address) and removes
duplicate data.  The UART nodes were extended via full path and phandle
which makes it difficult to review and spot actual differences.

No functional change (no difference in dtx_diff).
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>