dt-bindings: gpio: document the new pull-up/pull-down flags

This commit extends the flags that can be used in GPIO specifiers to
indicate if a pull-up resistor or pull-down resistor should be
enabled.

It is the backport of linux commit ede033e1e863c ('dt-bindings:
gpio: document the new pull-up/pull-down flags')
from Thomas Petazzoni <thomas.petazzoni@bootlin.com>
and integrated in v5.1-rc1
https://github.com/torvalds/linux/commit/ede033e1e863c

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/dt-bindings/gpio/gpio.h

@@ -33,4 +33,10 @@
 #define GPIO_PERSISTENT 0
 #define GPIO_TRANSITORY 8
 
+/* Bit 4 express pull up */
+#define GPIO_PULL_UP 16
+
+/* Bit 5 express pull down */
+#define GPIO_PULL_DOWN 32
+
 #endif