arm64: mm: remove unnecessary BUG_ON

The memblock_alloc() and memblock_alloc_base() will panic on their own
if no free memory, remove pointless BUG_ON.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>