google/cyan: convert to variant configuration

Setup cyan to be the baseboard for other Google Braswell
boards, to be added in subsequent commits:

- Keep code common to all Google Braswell boards in the baseboard,
and separate out the board-specific bits into the new cyan variant.

- Define the I2C ACPI devices such that they can be easily reused for
other variants.

- Switch the trackpad/touchscreen interrupts from edge to level,
for better performance/compatibility, as was done with all previous
Google boards.

- Add code to the baseboard to allow optional variant-specific
parameters to be used for both memory and silicon init.

- Remove superfluous includes, replace some hardcoded values with
variables, and correct typos/formatting errors.

Change-Id: Iabbbad16efa9cfa79338f4e94d0771779900d8d9
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/21126
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/mainboard/google/cyan/smihandler.c

@@ -25,7 +25,7 @@
 #include <soc/pm.h>
 #include <soc/gpio.h>
 
-#include "onboard.h"
+#include <variant/onboard.h>
 
 /* The wake gpio is SUS_GPIO[0]. */
 #define WAKE_GPIO_EN SUS_GPIO_EN0