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>

doc/device-tree-bindings/gpio/gpio.txt

@@ -65,6 +65,30 @@ Example of a node using GPIOs:
 GPIO_ACTIVE_HIGH is 0, so in this example gpio-specifier is "18 0" and encodes
 GPIO pin number, and GPIO flags as accepted by the "qe_pio_e" gpio-controller.
 
+Optional standard bitfield specifiers for the last cell:
+
+- Bit 0: 0 means active high, 1 means active low
+- Bit 1: 0 mean push-pull wiring, see:
+           https://en.wikipedia.org/wiki/Push-pull_output
+         1 means single-ended wiring, see:
+           https://en.wikipedia.org/wiki/Single-ended_triode
+- Bit 2: 0 means open-source, 1 means open drain, see:
+           https://en.wikipedia.org/wiki/Open_collector
+- Bit 3: 0 means the output should be maintained during sleep/low-power mode
+         1 means the output state can be lost during sleep/low-power mode
+- Bit 4: 0 means no pull-up resistor should be enabled
+         1 means a pull-up resistor should be enabled
+         This setting only applies to hardware with a simple on/off
+         control for pull-up configuration. If the hardware has more
+         elaborate pull-up configuration, it should be represented
+         using a pin control binding.
+- Bit 5: 0 means no pull-down resistor should be enabled
+         1 means a pull-down resistor should be enabled
+         This setting only applies to hardware with a simple on/off
+         control for pull-down configuration. If the hardware has more
+         elaborate pull-down configuration, it should be represented
+         using a pin control binding.
+
 1.1) GPIO specifier best practices
 ----------------------------------
 

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