mb/google/brya/var/vell: increase RFI Spread Spectrum to 6%

Increase RFI Spread Spectrum to 6% for Vell as RF team request. The
default of Spread Spectrum in FSP is 1.5%, and set 1.5% in baseboard
as default.

BUG=b:228929196
TEST=emerge-brya coreboot and pass RF test as before

Change-Id: I7cdca8f51ad18f4ab03e4e6c744b60da68263ce2
Signed-off-by: Robert Chen <robert.chen@quanta.corp-partner.google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/63440
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>

src/mainboard/google/brya/variants/vell/overridetree.cb

@@ -71,6 +71,9 @@ chip soc/intel/alderlake
 	register "usb2_ports[6]" = "USB2_PORT_MID(OC_SKIP)"
 	register "sagv" = "SaGv_Enabled"
 
+	# FIVR RFI Spread Spectrum 6%
+	register "fivr_spread_spectrum" = "FIVR_SS_6"
+
 	# I2C Port Config
 	register "serial_io_i2c_mode" = "{
 		[PchSerialIoIndexI2C0] = PchSerialIoPci,