rockchip: kevin/gru: Slow memory down to 300 MHz

At the higher speeds stressapptest shows memory errors.  We don't want
to track down random problems due to simple memory corruption, so slow
memory back down to 300 MHz until someone figures out how to make it
faster without sacrificing reliability.

BRANCH=None
BUG=chrome-os-partner:54144
TEST=stressapptest -M 1024 -s 240

Change-Id: I02182b25e677e27e8541445938f9da9ae9553fa6
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/350480
Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>

src/mainboard/google/gru/sdram_inf/gru-sdram-lpddr3-hynix-4GB.inc

@@ -52,7 +52,7 @@
 			}
 		}
 	},
-	.ddr_freq = 928*MHz,
+	.ddr_freq = 300*MHz,
 	.dramtype = LPDDR3,
 	.num_channels = 2,
 	.stride = 13,

src/mainboard/google/gru/sdram_inf/kevin-sdram-lpddr3-hynix-4GB.inc

@@ -51,7 +51,7 @@
 			}
 		}
 	},
-	.ddr_freq = 666*MHz,
+	.ddr_freq = 300*MHz,
 	.dramtype = LPDDR3,
 	.num_channels = 2,
 	.stride = 13,