mainboard/google/zoombini: enable 4 cores

BUG=b:70731385
BRANCH=master
TEST='emerge-meowth coreboot chromeos-bootimage', flash
image.serial.bin to meowth board, boot into kernel, and
verify 4 cores are running.

Change-Id: Ia233e41acd19b317f82433a5d41d84ea934a66c4
Signed-off-by: Nick Vaccaro <nvaccaro@chromium.org>
Reviewed-on: https://review.coreboot.org/23839
Reviewed-by: Nick Vaccaro <nvaccaro@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/mainboard/google/zoombini/Kconfig

@@ -77,6 +77,10 @@ config INCLUDE_SND_MAX98373_NHLT
 	select NHLT_DMIC_2CH_16B
 	select NHLT_MAX98373
 
+config MAX_CPUS
+	int
+	default 4
+
 config VARIANT_DIR
 	string
 	default "meowth" if BOARD_GOOGLE_MEOWTH