riscv: Change phys_addr_t and phys_size_t to 64-bit

phys_addr_t and phys_size_t are currently defined as `unsigned long`,
but RV32 supports 34-bit physical address, hence both phys_addr_t and
phys_size_t should be defined to 64-bit using `unsigned long long`.

Signed-off-by: Bin Meng <bin.meng@windriver.com>