nyan*: Reduce the EC SPI bus frequency to 3 MHz.

The EC doesn't seem to be able to handle its bus running at 4 MHz or higher.
To avoid it not being able to keep up, we reduce the frequency of that bus on
all nyan derivatives to 3 MHz. Because PLLP can't be divided that low, we
switch the clock source to CLKM.

BUG=chrome-os-partner:22849
TEST=Built and booted on nyan.
BRANCH=None

Change-Id: I8f31b41098d64634427b4686f5333012f643fada
Signed-off-by: Gabe Black <gabeblack@google.com>
Reviewed-on: https://chromium-review.googlesource.com/193349
Commit-Queue: Gabe Black <gabeblack@chromium.org>
Tested-by: Gabe Black <gabeblack@chromium.org>
Reviewed-by: Gabe Black <gabeblack@chromium.org>

src/mainboard/google/nyan/romstage.c

@@ -129,7 +129,7 @@ static void setup_pinmux(void)
 
 static void configure_ec_spi_bus(void)
 {
-	clock_configure_source(sbc1, PLLP, 5000);
+	clock_configure_source(sbc1, CLK_M, 3000);
 }
 
 static void configure_tpm_i2c_bus(void)

src/mainboard/google/nyan_big/romstage.c

@@ -129,7 +129,7 @@ static void setup_pinmux(void)
 
 static void configure_ec_spi_bus(void)
 {
-	clock_configure_source(sbc1, PLLP, 5000);
+	clock_configure_source(sbc1, CLK_M, 3000);
 }
 
 static void configure_tpm_i2c_bus(void)

src/mainboard/google/nyan_blaze/romstage.c

@@ -129,7 +129,7 @@ static void setup_pinmux(void)
 
 static void configure_ec_spi_bus(void)
 {
-	clock_configure_source(sbc1, PLLP, 5000);
+	clock_configure_source(sbc1, CLK_M, 3000);
 }
 
 static void configure_tpm_i2c_bus(void)