arm64: Refine comment about size of struct pmap_large_md_page

pmap_init_pv_table does not assume a specific size of 64.  It does
assume a size that evenly divides the size of a page since it inserts
pages from different domains into the kva backing the global pv_table
array.

Reviewed by:	imp, markj
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D48455

sys/arm64/arm64/pmap.c

@@ -1547,11 +1547,11 @@ pmap_init_pv_table(void)
 	int domain, i, j, pages;
 
 	/*
-	 * We strongly depend on the size being a power of two, so the assert
-	 * is overzealous. However, should the struct be resized to a
-	 * different power of two, the code below needs to be revisited.
+	 * We depend on the size being evenly divisible into a page so
+	 * that the pv_table array can be indexed directly while
+	 * safely spanning multiple pages from different domains.
 	 */
-	CTASSERT((sizeof(*pvd) == 64));
+	CTASSERT(PAGE_SIZE % sizeof(*pvd) == 0);
 
 	/*
 	 * Calculate the size of the array.