Added nrf51 ac variant

cpu/nrf51/ldscripts/nrf51x22xxac.ld

+/*
+ * Copyright (C) 2015 Freie Universität Berlin
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @addtogroup      cpu_nrf51822
+ * @{
+ *
+ * @file
+ * @brief           Memory definitions for the NRF51X22XXAC
+ *
+ * @author          Stephane D'Alu <sdalu@sdalu.com>
+ *
+ * @}
+ */
+
+MEMORY
+{
+    rom (rx)    : ORIGIN = 0x00000000, LENGTH = 256K
+    ram (rwx)   : ORIGIN = 0x20000000, LENGTH = 32K
+}
+
+INCLUDE cortexm_base.ld