power_key: adjust devfreq sysfs naming to linux 5.6

Linux changes the sysfs naming of devfreq from what we currently use, see
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=66d0e797bf095d407479c89952d42b1d96ef0a7f



Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>

usr/bin/power_key.py

@@ -21,8 +21,19 @@ def check_screen():
         return True
 
 
+def find_dram_devfreq():
+     for node in ['devfreq0', '3d400000.memory-controller']:
+         dram = "/sys/class/devfreq/{}".format(node)
+         if os.path.isdir(dram):
+             break
+     return dram
+
+
+set_freq = os.path.join(find_dram_devfreq(), 'userspace/set_freq')
+
+
 def set_dram_freq(freq):
-    speedF = open("/sys/class/devfreq/devfreq0/userspace/set_freq", "w+")
+    speedF = open(set_freq, "w+")
     speedF.write(freq + "\n")
     speedF.close()
 
@@ -40,7 +51,7 @@ def main(args):
     foundpath = ''.join(path)
     dev = InputDevice(foundpath)
 
-    f = open("/sys/class/devfreq/devfreq0/governor", "w+")
+    f = open(os.path.join(find_dram_devfreq(), 'governor'), "w+")
     f.write("userspace\n")
     f.close()