gemini: Make root filesystem writeable

The squashfs+jffs2 root filesystem only came up as read-only
and the config would not take:

[   25.600237] mount_root: Could not open mtd device: /dev/mtd4
[   25.634674] mount_root: reading rootfs_data failed
[   25.665346] mount_root: Could not open mtd device: /dev/mtd3
[   25.699802] mount_root: reading rootfs failed
[   25.726426] mount_root: mounting /dev/root

Well that is not very strange since the whole firmware partition
is made read-only in the device tree. Let's fix that by augmenting
the OpenWrt patch.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

target/linux/gemini/patches-4.19/0020-ARM-dts-Augment-DIR-685-partition-table-for-OpenWrt.patch

@@ -1,7 +1,7 @@
-From 854934b0ce8e758ce581f5ddbc30e618ab46fbdb Mon Sep 17 00:00:00 2001
+From ea99e7570882df53478f9b5fa5886a6e92fc2c7f Mon Sep 17 00:00:00 2001
 From: Linus Walleij <linus.walleij@linaro.org>
 Date: Mon, 11 Mar 2019 15:44:29 +0100
-Subject: [PATCH 2/2] ARM: dts: Augment DIR-685 partition table for OpenWrt
+Subject: [PATCH] ARM: dts: Augment DIR-685 partition table for OpenWrt
 
 Rename the firmware partition so that the firmware MTD
 splitter will do its job, drop the rootfs arguments as
@@ -9,8 +9,8 @@ the MTD splitter will set this up automatically.
 
 Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
 ---
- arch/arm/boot/dts/gemini-dlink-dir-685.dts | 5 +++--
- 1 file changed, 3 insertions(+), 2 deletions(-)
+ arch/arm/boot/dts/gemini-dlink-dir-685.dts | 6 +++---
+ 1 file changed, 3 insertions(+), 3 deletions(-)
 
 --- a/arch/arm/boot/dts/gemini-dlink-dir-685.dts
 +++ b/arch/arm/boot/dts/gemini-dlink-dir-685.dts
@@ -23,7 +23,7 @@ Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
  		stdout-path = "uart0:19200n8";
  	};
  
-@@ -286,7 +286,8 @@
+@@ -286,9 +286,9 @@
  				 * this is called "upgrade" on the vendor system.
  				 */
  				partition@40000 {
@@ -31,5 +31,7 @@ Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
 +					compatible = "wrg";
 +					label = "firmware";
  					reg = <0x00040000 0x01f40000>;
- 					read-only;
+-					read-only;
  				};
+ 				/* RGDB, Residental Gateway Database? */
+ 				partition@1f80000 {