arm64/mm: Poison initmem while freeing with free_reserved_area()

Platform implementation for free_initmem() should poison the memory while
freeing it up. Hence pass across POISON_FREE_INITMEM while calling into
free_reserved_area(). The same is being followed in the generic fallback
for free_initmem() and some other platforms overriding it.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-kernel@vger.kernel.org
Reviewed-by: Steven Price <steven.price@arm.com>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>