ath79: add support for kernel 5.4

Signed-off-by: David Bauer <mail@david-bauer.net>
[refreshed]
Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>

* Sync the patches with the changes done for kernel 4.19
* Use KERNEL_TESTING_PATCHVER
* Refresh the configuration
* Fix multiple compile bugs in the patches
* Only add own ag71xx files for kernel 4.19 and use upstream version for
  5.4.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

target/linux/ath79/patches-5.4/0002-watchdog-ath79-fix-maximum-timeout.patch

@@ -0,0 +1,32 @@
+From 5f5c9858af167f842ee8df053920b98387a71af1 Mon Sep 17 00:00:00 2001
+From: John Crispin <john@phrozen.org>
+Date: Mon, 5 Mar 2018 11:41:25 +0100
+Subject: [PATCH 02/27] watchdog: ath79: fix maximum timeout
+
+If the userland tries to set a timeout higher than the max_timeout,
+then we should fallback to max_timeout.
+
+Signed-off-by: John Crispin <john@phrozen.org>
+---
+ drivers/watchdog/ath79_wdt.c | 8 ++++++--
+ 1 file changed, 6 insertions(+), 2 deletions(-)
+
+--- a/drivers/watchdog/ath79_wdt.c
++++ b/drivers/watchdog/ath79_wdt.c
+@@ -111,10 +111,14 @@ static inline void ath79_wdt_disable(voi
+ 
+ static int ath79_wdt_set_timeout(int val)
+ {
+-	if (val < 1 || val > max_timeout)
++	if (val < 1)
+ 		return -EINVAL;
+ 
+-	timeout = val;
++	if (val > max_timeout)
++		timeout = max_timeout;
++	else
++		timeout = val;
++
+ 	ath79_wdt_keepalive();
+ 
+ 	return 0;