chips/samd21: Hide definitions of missing timers

samd21g18a, which is used by all of the current samd21 ports, doesn't
have tc6, tc7 or tcc3. Remove the definitions to keep ports from
trying to use them.

Signed-off-by: Keith Packard <keithp@keithp.com>

chips/samd21/samd21.h

@@ -849,11 +849,14 @@ extern struct samd21_tc samd21_tc4;
 extern struct samd21_tc samd21_tc5;
 #define samd21_tc5 (*(struct samd21_tc *) 0x42003400)
 
+#ifdef ATSAMD21J
+/* Present on all of the samd21j parts and the samd21g16l */
 extern struct samd21_tc samd21_tc6;
 #define samd21_tc6 (*(struct samd21_tc *) 0x42003800)
 
 extern struct samd21_tc samd21_tc7;
 #define samd21_tc7 (*(struct samd21_tc *) 0x42003c00)
+#endif
 
 #define SAMD21_TC_CTRLA_SWRST		0
 #define SAMD21_TC_CTRLA_ENABLE		1
@@ -957,8 +960,11 @@ extern struct samd21_tcc samd21_tcc1;
 extern struct samd21_tcc samd21_tcc2;
 #define samd21_tcc2 (*(struct samd21_tcc *) 0x42002800)
 
+#ifdef SAMD21E17D
+/* only on the samd21e17d */
 extern struct samd21_tcc samd21_tcc3;
 #define samd21_tcc3 (*(struct samd21_tcc *) 0x42006000)
+#endif
 
 #define SAMD21_TCC_CTRLA_SWRST		0
 #define SAMD21_TCC_CTRLA_ENABLE		1