arm/arm64: Use pgd_alloc() to allocate mmu_idmap

Until commit 031755db ("arm: enable vmalloc"), the idmap was allocated
using pgd_alloc(). After that commit, all the page table allocator
functions were switched to using the page allocator, but pgd_alloc() was
left unchanged and became unused, with the idmap now being allocated with
alloc_page().

For arm64, the pgd table size varies based on the page size, which is
configured by the user. For arm, it will always contain 4 entries (it
translates bits 31:30 of the input address). To keep things simple and
consistent with the other functions and across both architectures, modify
pgd_alloc() to use alloc_page() instead of memalign like the rest of the
page table allocator functions and use it to allocate the idmap.

Note that when the idmap is created, alloc_ops->memalign is
memalign_pages() which allocates memory with page granularity. Using
memalign() as before would still have allocated a full page.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>