Document: add MAX_IO_BLOCK_DEVICES platform macro

Add MAX_IO_BLOCK_DEVICES in porting guide. It's necessary to define
this macro to support io block device. With this macro, multiple
block devices could be opened at the same time. Each block device
stores its own state.
Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>

docs/porting-guide.md

@@ -448,6 +448,14 @@ must also be defined:
     Defines the maximum number of open IO handles. Attempting to open more IO
     entities than this value using `io_open()` will fail with -ENOMEM.
 
+*   **#define : MAX_IO_BLOCK_DEVICES**
+
+    Defines the maximum number of registered IO block devices. Attempting to
+    register more devices this value using `io_dev_open()` will fail
+    with -ENOMEM. MAX_IO_BLOCK_DEVICES should be less than MAX_IO_DEVICES.
+    With this macro, multiple block devices could be supported at the same
+    time.
+
 If the platform needs to allocate data within the per-cpu data framework in
 BL31, it should define the following macro. Currently this is only required if
 the platform decides not to use the coherent memory section by undefining the