Add a new variant of the of_parse_phandle_with_args() which treats the
cells name as optional. If it's missing, it is assumed that the phandle
has no arguments.

Up until now, a nvmem node didn't have any arguments, so all the device
trees haven't any '#*-cells' property. But there is a need for an
additional argument for the phandle, for which we need a '#*-cells'
property. Therefore, we need to support nvmem nodes with and without
this property.

Signed-off-by: Michael Walle <redacted>
---
 drivers/of/base.c  | 23 +++++++++++++++++++++++
 include/linux/of.h | 12 ++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/drivers/of/base.c b/drivers/of/base.c
index 5b907600f5b0..fb28bb26276e 100644
--- a/drivers/of/base.c
+++ b/drivers/of/base.c

@@ -1543,6 +1543,29 @@ int of_parse_phandle_with_args(const struct device_node *np, const char *list_na
 }
 EXPORT_SYMBOL(of_parse_phandle_with_args);
 
+/**
+ * of_parse_phandle_with_optional_args() - Find a node pointed by phandle in a list
+ *
+ * Same as of_parse_phandle_args() except that if the cells_name property is
+ * not found, cell_count of 0 is assumed.
+ *
+ * This is used to useful, if you have a phandle which didn't have arguments
+ * before and thus doesn't have a '#*-cells' property but is now migrated to
+ * having arguments while retaining backwards compatibility.
+ */
+int of_parse_phandle_with_optional_args(const struct device_node *np,
+					const char *list_name,
+					const char *cells_name, int index,
+					struct of_phandle_args *out_args)
+{
+	if (index < 0)
+		return -EINVAL;