reset: add no-op stubs for optional reset control

My motivation for this patch is to make reset control handling
optional for generic drivers.

I want to add reset control to drivers/usb/host/ehci-generic.c,
but it is used by several platforms, some will implement a reset
controller driver, some will not.

Add no-op stubs in order to avoid link error for drivers that
implement reset controlling, but still it is optional.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

include/reset.h

@@ -7,6 +7,8 @@
 #ifndef _RESET_H
 #define _RESET_H
 
+#include <linux/errno.h>
+
 /**
  * A reset is a hardware signal indicating that a HW module (or IP block, or
  * sometimes an entire off-CPU chip) reset all of its internal state to some
@@ -58,6 +60,7 @@ struct reset_ctl {
 	unsigned long id;
 };
 
+#ifdef CONFIG_DM_RESET
 /**
  * reset_get_by_index - Get/request a reset signal by integer index.
  *
@@ -132,4 +135,33 @@ int reset_assert(struct reset_ctl *reset_ctl);
  */
 int reset_deassert(struct reset_ctl *reset_ctl);
 
+#else
+static inline int reset_get_by_index(struct udevice *dev, int index,
+				     struct reset_ctl *reset_ctl)
+{
+	return -ENOTSUPP;
+}
+
+static inline int reset_get_by_name(struct udevice *dev, const char *name,
+				    struct reset_ctl *reset_ctl)
+{
+	return -ENOTSUPP;
+}
+
+static inline int reset_free(struct reset_ctl *reset_ctl)
+{
+	return 0;
+}
+
+static inline int reset_assert(struct reset_ctl *reset_ctl)
+{
+	return 0;
+}
+
+static inline int reset_deassert(struct reset_ctl *reset_ctl)
+{
+	return 0;
+}
+#endif
+
 #endif