dev: Set HDLCD default pixel clock for 1080p @ 60Hz

This patch changes the default pixel clock to effectively generate
1080p resolution at 60 frames per second. It is dependent upon the
kernel device tree file using the specified resolution / display
string in the comments.

src/dev/arm/RealView.py

@@ -165,9 +165,12 @@ class Pl111(AmbaDmaDevice):
 class HDLcd(AmbaDmaDevice):
     type = 'HDLcd'
     cxx_header = "dev/arm/hdlcd.hh"
-    pixel_clock = Param.Clock('65MHz', "Clock frequency of the pixel clock "
-                                       "(i.e. PXLREFCLK / OSCCLK 5; 23.75MHz "
-                                       "default up to 165MHz)")
+    # For reference, 1024x768MR-16@60  ~= 56 MHz
+    #                1920x1080MR-16@60 ~= 137 MHz
+    #                3840x2160MR-16@60 ~= 533 MHz
+    # Match against the resolution selected in the Linux DTS/DTB file.
+    pixel_clock = Param.Clock('137MHz', "Clock frequency of the pixel clock "
+                                        "(i.e. PXLREFCLK / OSCCLK 5")
     vnc = Param.VncInput(Parent.any, "Vnc server for remote frame buffer "
                                      "display")
     amba_id = 0x00141000