Synchronize MdePkg/Library *.h files with c files

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@6985 6f19259b-4bc3-4df7-8a09-765794883524

MdePkg/Library/PeiIoLibCpuIo/IoLib.c

@@ -337,6 +337,8 @@ MmioRead8 (
 
   @param  Address The MMIO register to write.
   @param  Value   The value to write to the MMIO register.
+  
+  @return Value.
 
 **/
 UINT8
@@ -404,6 +406,8 @@ MmioRead16 (
 
   @param  Address The MMIO register to write.
   @param  Value   The value to write to the MMIO register.
+  
+  @return Value.
 
 **/
 UINT16
@@ -474,6 +478,8 @@ MmioRead32 (
 
   @param  Address The MMIO register to write.
   @param  Value   The value to write to the MMIO register.
+  
+  @return Value.
 
 **/
 UINT32