x86: tangier: Reserve PCI ECAM in motherboard resources

Per PCI firmware specification the ACPI has to reserve the memory
which is defined as PCI ECAM.

Fixes: 39665bee ("x86: tangier: Enable ACPI support for Intel Tangier")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>