arm64/mm: Implement pXX_leaf_size() support

ARM64 has non-pagetable aligned large page support with PTE_CONT, when
this bit is set the page is part of a super-page. Match the hugetlb
code and support these super pages for PTE and PMD levels.

This enables PERF_SAMPLE_{DATA,CODE}_PAGE_SIZE to report accurate
pagetable leaf sizes.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lkml.kernel.org/r/20201126125747.GG2414@hirez.programming.kicks-ass.net