Add the common extra.ld.S and customized rk3399.ld.S to extend
to more features for different platforms.
For example, we can add SRAM section and specific address to
load there if we need it, and the common bl31.ld.S not need to
be modified.

Therefore, we can remove the unused codes which copying explicitly
from the function pmusram_prepare(). It looks like more clear.

Change-Id: Ibffa2da5e8e3d1d2fca80085ebb296ceb967fce8
Signed-off-by: Xing Zheng <zhengxing@rock-chips.com>
Signed-off-by: Caesar Wang <wxt@rock-chips.com>