cpu/sam0_common: hook up low power RAM as extra heap

This enables unused low-lower RAM on saml21/samd5x to be used for
heap memory.

cpu/cortexm_common/ldscripts/cortexm_base.ld

@@ -185,4 +185,9 @@ SECTIONS
         /* Round size so that we can use 4 byte copy in init */
         . = ALIGN(4);
     } > bkup-ram
+
+    .heap3 (NOLOAD) : ALIGN(4) {
+        _sheap1 = . ;
+        _eheap1 = ORIGIN(bkup-ram) + LENGTH(bkup-ram);
+    } > bkup-ram
 }

cpu/sam0_common/include/cpu_conf.h

@@ -90,6 +90,13 @@ as shown in the NVM Row Organization figure. */
 #endif
 /** @} */
 
+/**
+ * @brief   The CPU has Low Power RAM that can be used as Heap
+ */
+#ifdef CPU_HAS_BACKUP_RAM
+#define NUM_HEAPS (2)
+#endif
+
 #ifdef __cplusplus
 }
 #endif