mb/google/fizz: Provide baseboard and variant concepts

In order to be able to share code across different fizz variants,
provide the concept of baseboard and variants. New directory layout:

variants/baseboard - code
variants/baseboard/include/baseboard - headers
variants/fizz - code
variants/fizz/include/variant - headers

New boards would then add themselves under their board name within
"variants" directory.

This is purely an organizational change.

BUG=b:117066935
BRANCH=Fizz
TEST=emerge-fizz coreboot
CQ-DEPEND=CL:1273514

Change-Id: I28cc41681e7af88ddeba2e847dc0a4686606feb2
Signed-off-by: David Wu <david_wu@quanta.corp-partner.google.com>
Reviewed-on: https://review.coreboot.org/28962
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/mainboard/google/fizz/mainboard.c

@@ -20,7 +20,7 @@
 #include <ec/ec.h>
 #include <ec/google/chromeec/ec.h>
 #include <gpio.h>
-#include <mainboard/google/fizz/gpio.h>
+#include <variant/gpio.h>
 #include <smbios.h>
 #include <soc/gpio.h>
 #include <soc/pci_devs.h>