armv7m: cache: add invalidate_icache_all() stub

This commit:
d409c96216
causes build failure with ICACHE enabled. This is due to missing
invalidate_icache_all() stub. Let's add empty invalidate_icache_all() in
the case where ICACHE is not enabled.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>

arch/arm/cpu/armv7m/cache.c

@@ -332,6 +332,11 @@ void icache_disable(void)
 	isb();	/* subsequent instructions fetch see cache disable effect */
 }
 #else
+void invalidate_icache_all(void)
+{
+	return;
+}
+
 void icache_enable(void)
 {
 	return;