arm64: fix build warning for ARM64_MEMSTART_SHIFT

arm64: fix build warning for ARM64_MEMSTART_SHIFT

When building with W=1, the following warning occurs.

arch/arm64/include/asm/kernel-pgtable.h:129:41: error: "PUD_SHIFT" is not defined, evaluates to 0 [-Werror=undef]
  129 | #define ARM64_MEMSTART_SHIFT            PUD_SHIFT
      |                                         ^~~~~~~~~
arch/arm64/include/asm/kernel-pgtable.h:142:5: note: in expansion of macro ‘ARM64_MEMSTART_SHIFT’
  142 | #if ARM64_MEMSTART_SHIFT < SECTION_SIZE_BITS
      |     ^~~~~~~~~~~~~~~~~~~~

The generic PUD_SHIFT was defined in include/asm-generic/pgtable-nopud.h,
however the #ifndef __ASSEMBLY__ guard in this header file makes it unavailable
for assembly files. While someone .S file include the <asm/kernel-pgtable.h>,
the build warning would occur. Now move the macro ARM64_MEMSTART_SHIFT and
ARM64_MEMSTART_ALIGN to arch/arm64/mm/init.c where it is used only, to avoid
this issue.

Signed-off-by: Zhang Jianhua <chris.zjh@huawei.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20230804075615.3334756-1-chris.zjh@huawei.com
Signed-off-by: Will Deacon <will@kernel.org>