Revise heap and stack size for Ethernet of GR-PEACH

I changed stack size because the deafult stack size may not be enough for
Ethernet of GR-PEACH(Cortex-A).
- Stack size is default size + 128 byte
In order to maximize the performance of Ethernet, I changed heap size to
below.
- Heap size is default size * 32 byte(send descriptor num is 16 and recv
descriptoer num is 16).

Author: TomoYamanaka

features/lwipstack/mbed_lib.json

@@ -127,6 +127,12 @@
         },
         "EFM32GG11-STK3701": {
             "mem-size": 36560
+        },
+        "RZ_A1_EMAC": {
+            "tcpip-thread-stacksize": 1328,
+            "default-thread-stacksize": 640,
+            "ppp-thread-stacksize": 896,
+            "mem-size": 51200
         }
     }
 }