x86/amd_nb: Add northbridge support for Hygon family 18h model 4h

hygon inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I7DX6V


CVE: NA

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Add dedicated functions to initialize the northbridge for Hygon family
18h model 4h processors.

Signed-off-by: Pu Wen <puwen@hygon.cn>