Somehow HSS needs to access address 0 [1] for the DDR calibration data
which is in the chipset's reserved memory. Let's map it.

[1] See the config_copy() calls in various places in ddr_setup() in
    the HSS source codes.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 1603863010-15807-9-git-send-email-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>