Kernel/PCI: Introduce a new ECAM access mechanism

Now the kernel supports 2 ECAM access methods.
MMIOAccess was renamed to WindowedMMIOAccess and is what we had until
now - each device that is detected on boot is assigned to a
memory-mapped window, so IO operations on multiple devices can occur
simultaneously due to creating multiple virtual mappings, hence the name
is a memory-mapped window.

This commit adds a new class called MMIOAccess (not to be confused with
the old MMIOAccess class). This class creates one memory-mapped window.
On each IO operation on a configuration space of a device, it maps the
requested PCI bus region to that window. Therefore it holds a SpinLock
during the operation to ensure that no other PCI bus region was mapped
during the call.

A user can choose to either use PCI ECAM with memory-mapped window
for each device, or for an entire bus. By default, the kernel prefers to
map the entire PCI bus region.

Kernel/PCI/Definitions.h

@@ -226,6 +226,7 @@ PhysicalID get_physical_id(Address address);
 
 class Access;
 class MMIOAccess;
+class WindowedMMIOAccess;
 class IOAccess;
 class MMIOSegment;
 class DeviceController;