Some peripherals require 64-bit PCI address, so let's map the high
mmio space for PCIe.

For RV32, the address is hardcoded to below 4 GiB from the highest
accessible physical address. For RV64, the base address depends on
top of RAM and is aligned to its size which is using 16 GiB for now.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210220144807.819-5-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>