core/memory: Create a special MapMemoryRegion for physical memory.

This allows us to create a fastmem arena within the memory.cpp helpers.

src/core/memory.h

@@ -19,8 +19,9 @@ class System;
 }
 
 namespace Kernel {
+class PhysicalMemory;
 class Process;
-}
+} // namespace Kernel
 
 namespace Memory {
 
@@ -65,6 +66,19 @@ public:
      */
     void SetCurrentPageTable(Kernel::Process& process);
 
+    /**
+     * Maps an physical buffer onto a region of the emulated process address space.
+     *
+     * @param page_table The page table of the emulated process.
+     * @param base       The address to start mapping at. Must be page-aligned.
+     * @param size       The amount of bytes to map. Must be page-aligned.
+     * @param memory     Physical buffer with the memory backing the mapping. Must be of length
+     *                   at least `size + offset`.
+     * @param offset     The offset within the physical memory. Must be page-aligned.
+     */
+    void MapMemoryRegion(Common::PageTable& page_table, VAddr base, u64 size,
+                         Kernel::PhysicalMemory& memory, VAddr offset);
+
     /**
      * Maps an allocated buffer onto a region of the emulated process address space.
      *