powerpc/book3s64/hugetlb: Fix disabling hugetlb when fadump is active

Commit 85975387 ("powerpc/fadump: Do not use hugepages when fadump
is active") disabled hugetlb support when fadump is active by returning
early from hugetlbpage_init():arch/powerpc/mm/hugetlbpage.c and not
populating hpage_shift/HPAGE_SHIFT.

Later, commit 2354ad25 ("powerpc/mm: Update default hugetlb size
early") moved the allocation of hpage_shift/HPAGE_SHIFT to early boot,
which inadvertently re-enabled hugetlb support when fadump is active.

Fix this by implementing hugepages_supported() on powerpc. This ensures
that disabling hugetlb for the fadump kernel is independent of
hpage_shift/HPAGE_SHIFT.

Fixes: 2354ad25 ("powerpc/mm: Update default hugetlb size early")
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20241217074640.1064510-1-sourabhjain@linux.ibm.com