arm64: fix data type for physical address

Use phys_addr_t for physical address in alloc_init_pud. Although
phys_addr_t and unsigned long are 64 bit in arm64, it is better
to use phys_addr_t to describe physical addresses.

Signed-off-by: Min-Hua Chen <orca.chen@gmail.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>