cc26x0: Adjust ldscript memory segment attributes

cpu/cc26x0/ldscripts/cc26x0f128.ld

@@ -11,9 +11,9 @@
 /* Memory Space Definitions: */
 MEMORY
 {
-    rom  (rx ) : ORIGIN = 0x00000000, LENGTH = 0x00020000 - 88 /* technically, it's 128K */
-    gpram  (rwx) : ORIGIN = 0x11000000, LENGTH = 8K /* configurable as cache. 20K here, 8K there, and 2K in the ld-script of cc26x0ware */
-    ram    (rwx) : ORIGIN = 0x20000000, LENGTH = 20K /* sram */
+    rom (rx)   : ORIGIN = 0x00000000, LENGTH = 0x00020000 - 88 /* technically, it's 128K */
+    gpram      : ORIGIN = 0x11000000, LENGTH = 8K /* configurable as cache. 20K here, 8K there, and 2K in the ld-script of cc26x0ware */
+    ram (w!rx) : ORIGIN = 0x20000000, LENGTH = 20K /* sram */
 }
 
 INCLUDE cortexm_base.ld