x86: Rename MMCONF_BASE_ADDRESS and make it common across x86

This setting will be used by more than just ivybridge so make it common.

Also rename it to PCIE_ECAM_BASE which is a more descriptive name.

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