arm64: Fix memory shareability attribute for ioremap_wc/cache

Write-combine and cacheable mappings use Normal memory on arm64. On SMP
systems, the pte needs the shareability bit which is set in
pgprot_default. Use this for defining PROT_DEFAULT used by ioremap_wc
and ioremap_cache (Device memory is shareable by default, does not need
additional attributes).

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>