hw/riscv: microchip_pfsoc: Document where to look at the SoC memory maps

hw/riscv: microchip_pfsoc: Document where to look at the SoC memory maps

It is not easy to find out the memory map for a specific component
in the PolarFire SoC as the information is scattered in different
documents. Add some comments so that people can know where to get
such information from the Microchip website.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 1603863010-15807-2-git-send-email-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>