console: Use map_sysmem() for the pre-relocation console

At present this feature casts the address to a pointer. Use the
map_sysmem() function so that it will work correctly on sandbox.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Stephen Warren <swarren@nvidia.com>