rockchip/rk3399: enable DPLL SSC for DDR EMI test on bob

Spread Spectrum Modulator (SSMOD) is a fully-digital circuit used to
modulate the frequency of the Silicon Creations’ Fractional PLL in order
to reduce EMI.

We need to turn the DPLL spread spectrum feature on to
reduce the EMI noise for DDR on bob.

Change-Id: I75461d4235bcf55324e6664a1220754e770b4786
Signed-off-by: Xing Zheng <zhengxing@rock-chips.com>
Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Reviewed-on: https://review.coreboot.org/19557
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/mainboard/google/gru/Kconfig

@@ -38,6 +38,7 @@ config BOARD_SPECIFIC_OPTIONS
 	select MAINBOARD_HAS_CHROMEOS
 	select MAINBOARD_HAS_NATIVE_VGA_INIT
 	select RAM_CODE_SUPPORT
+	select RK3399_SPREAD_SPECTRUM_DDR if BOARD_GOOGLE_BOB
 	select RTC
 	select SOC_ROCKCHIP_RK3399
 	select SPI_FLASH