Libretro buildfix, add a comment

Core/MemMap.cpp

@@ -71,6 +71,11 @@ u8 *m_pUncachedKernelRAM[3];
 static u8 *m_pPhysicalVRAM[4];
 static u8 *m_pUncachedVRAM[4];
 
+// Hardware registers are mapped at 0xbc000000 and forwards (physical address 0x1C000000, but we need
+// the 0x80000000 kernel flag and 0x40000000 uncached flag for these). Exception vectors are at 0xbfc00000.
+// Since we do HLE emulation, we currently don't bother with any of that.
+// Some limited documentation here: https://www.psdevwiki.com/psp/Hardware_Registers#Introduction
+
 // Holds the ending address of the PSP's user space.
 // Required for HD Remasters to work properly.
 // This replaces RAM_NORMAL_SIZE at runtime.