dt-bindings: gpio: Add GPIO_PULL_DISABLE flag support

Currently, GPIO_PULL_DISABLE flag support is not yet implemented
on U-Boot side but it is on kernel side.

Add GPIO_PULL_DISABLE define to allow DT compilation when
kernel DT synchronization will be performed in the future.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Change-Id: Iafd7ca8d09d23003ede202e55f86b7e51f6c2cb8
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/u-boot/+/312499
ACI: CITOOLS <MDG-smet-aci-reviews@list.st.com>
ACI: CIBUILD <MDG-smet-aci-builds@list.st.com>
Domain-Review: Patrick DELAUNAY <patrick.delaunay@foss.st.com>

Author: Patrice Chotard

include/dt-bindings/gpio/gpio.h

@@ -39,4 +39,7 @@
 /* Bit 5 express pull down */
 #define GPIO_PULL_DOWN 32
 
+/* Bit 6 express pull disable */
+#define GPIO_PULL_DISABLE 64
+
 #endif