MdePkg Updated the comments of EFI_SYSTEM_TABLE and ReadKeyStroke

1. Refer to Uefi spec 2.10 section 4.3.1, Updated the comments of
EFI_SYSTEM_TABLE to satisfy ConIn/ConOut/StdErr requirements.
2. Refer to Uefi spec 2.10 section 13.3.3, Add a new retval
EFI_UNSUPPORTED to EFI_SIMPLE_TEXT_INPUT_EX_PROTOCOL.ReadKeyStrokeEx
and EFI_SIMPLE_TEXT_INPUT_PROTOCOL.ReadKeyStroke().

Signed-off-by: Junfeng Guan <junfengx.guan@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Zhiguang Liu <zhiguang.liu@intel.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>

MdePkg/Include/Protocol/SimpleTextIn.h

@@ -100,6 +100,7 @@ EFI_STATUS
   @retval EFI_NOT_READY    There was no keystroke data available.
   @retval EFI_DEVICE_ERROR The keystroke information was not returned due to
                            hardware errors.
+  @retval EFI_UNSUPPORTED  The device does not support the ability to read keystroke data.
 
 **/
 typedef