sam3: fix pedantic compiler warnings

cpu/sam3/lpm_arch.c

@@ -27,6 +27,7 @@ void lpm_arch_init(void)
 
 enum lpm_mode lpm_arch_set(enum lpm_mode target)
 {
+    (void) target;
     // TODO
     return 0;
 }

cpu/sam3/periph/random.c

@@ -40,7 +40,7 @@ int random_read(char *buf, unsigned int num)
 {
     /* cppcheck-suppress variableScope */
     uint32_t tmp;
-    int count = 0;
+    unsigned count = 0;
 
     while (count < num) {
         /* wait until new value is generated -> takes up to 84 cycles */