MIPS&LoongArch&NIOS2: Adjust prototypes of p?d_init()

LoongArch inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5OHOB



--------------------------------

We are preparing to add sparse vmemmap support to LoongArch. MIPS and
LoongArch need to call pgd_init()/pud_init()/pmd_init() when populating
page tables, so adjust their prototypes to make generic helpers can call
them.

NIOS2 declares pmd_init() but doesn't use, just remove it to avoid build
errors.

Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Feiyang Chen <chenfeiyang@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>