falco: Enable RTD2132 spread spectrum at 1.0%

This may need further tuning but will start at 1.0%.

boot on falco and check /sys/firmware/log

localhost ~ # grep RTD2132 /sys/firmware/log
RTD2132: Enable 1.0% Spread Spectrum
I2C: 01:35 (Realtek RTD2132 LVDS Bridge)

Change-Id: I96e1c14dbc6a7bfaf1c8deb1806c48bf2fd3e32a
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/61895
Reviewed-on: http://review.coreboot.org/4366
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>
Tested-by: build bot (Jenkins)

src/mainboard/google/falco/Kconfig

@@ -21,6 +21,7 @@ config BOARD_SPECIFIC_OPTIONS # dummy
 	select CACHE_ROM
 	select MARK_GRAPHICS_MEM_WRCOMB
 	select MONOTONIC_TIMER_MSR
+	select DRIVERS_I2C_RTD2132
 
 config VBOOT_RAMSTAGE_INDEX
 	hex

src/mainboard/google/falco/devicetree.cb

@@ -108,7 +108,13 @@ chip northbridge/intel/haswell
 				end
 			end # LPC bridge
 			device pci 1f.2 on end # SATA Controller
-			device pci 1f.3 on end # SMBus
+			device pci 1f.3 on # SMBus
+				chip drivers/i2c/rtd2132
+					# Enable Spread Sprectrum at 1.0%
+					register "sscg_percent" = "0x10"
+					device i2c 35 on end
+				end
+			end
 			device pci 1f.6 on end # Thermal
 		end
 	end