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>

arch/arm64/mm/mmu.c

@@ -77,7 +77,6 @@ static phys_addr_t __init early_pgtable_alloc(void)
 	void *ptr;
 
 	phys = memblock_alloc(PAGE_SIZE, PAGE_SIZE);
-	BUG_ON(!phys);
 
 	/*
 	 * The FIX_{PGD,PUD,PMD} slots may be in active use, but the FIX_PTE