dm: gpio: Rename set_dir_flags() method to update_flags()

The current method is a misnomer since it is also used (e.g. by stm32) to
update pull settings and open source/open drain.

Rename it and expand the documentation to cover a few more details.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>

Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
diff --git a/drivers/gpio/sandbox.c b/drivers/gpio/sandbox.c
index dc8d506..05f1792 100644
--- a/drivers/gpio/sandbox.c
+++ b/drivers/gpio/sandbox.c
@@ -177,8 +177,8 @@
 	return 0;
 }
 
-static int sb_gpio_set_dir_flags(struct udevice *dev, unsigned int offset,
-				 ulong flags)
+static int sb_gpio_set_flags(struct udevice *dev, unsigned int offset,
+			     ulong flags)
 {
 	ulong *dir_flags;
 
@@ -272,7 +272,7 @@
 	.set_value		= sb_gpio_set_value,
 	.get_function		= sb_gpio_get_function,
 	.xlate			= sb_gpio_xlate,
-	.set_dir_flags		= sb_gpio_set_dir_flags,
+	.set_flags		= sb_gpio_set_flags,
 	.get_dir_flags		= sb_gpio_get_dir_flags,
 #if CONFIG_IS_ENABLED(ACPIGEN)
 	.get_acpi		= sb_gpio_get_acpi,