refactor(mm): delete `arch_mm_clear_pa`

It has the same effect as `mm_round_down_to_page`.

Change-Id: Ic86f0ef427660168db83a9fcaa29338053f33dc7
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/src/mm.c b/src/mm.c
index 42e3810..9b2c006 100644
--- a/src/mm.c
+++ b/src/mm.c
@@ -435,7 +435,7 @@
 	mm_level_t root_level = mm_max_level(flags) + 1;
 	ptable_addr_t ptable_end = mm_ptable_addr_space_end(flags);
 	ptable_addr_t end = mm_round_up_to_page(pa_addr(pa_end));
-	ptable_addr_t begin = pa_addr(arch_mm_clear_pa(pa_begin));
+	ptable_addr_t begin = mm_round_down_to_page(pa_addr(pa_begin));
 	struct mm_page_table *root_table = &mm_page_table_from_pa(
 		ptable->root)[mm_index(begin, root_level)];