arm64: mm: Correct fixmap pagetable types

Compiling with STRICT_MM_TYPECHECKS gives the following
arch/arm64/mm/ioremap.c: In function ‘early_ioremap_init’:
arch/arm64/mm/ioremap.c:152:2: warning: passing argument 3 of
‘pud_populate’ from incompatible pointer type
  pud_populate(&init_mm, pud, bm_pmd);

The data types for bm_pmd and bm_pud are incorrectly set to pte_t.
This patch corrects these types.

Signed-off-by: Steve Capper <steve.capper@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>