rockchip: rk3399: Weaken dependency on SPL serial

Allow to disable serial console in SPL. Weak dependency is already used
with TPL serial.

Signed-off-by: Daniel Semkowicz <dse@thaumatec.com>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/arch/arm/mach-rockchip/Kconfig b/arch/arm/mach-rockchip/Kconfig
index fc1b638..269c219 100644
--- a/arch/arm/mach-rockchip/Kconfig
+++ b/arch/arm/mach-rockchip/Kconfig
@@ -257,7 +257,6 @@
 	select SPL_SYSCON if SPL
 	select TPL_NEEDS_SEPARATE_STACK if TPL
 	select SPL_SEPARATE_BSS
-	select SPL_SERIAL
 	select CLK
 	select FIT
 	select PINCTRL
@@ -288,6 +287,7 @@
 	imply SPL_DM_SEQ_ALIAS
 	imply SPL_FIT_SIGNATURE
 	imply SPL_ROCKCHIP_COMMON_BOARD
+	imply SPL_SERIAL
 	imply SYS_BOOTCOUNT_SINGLEWORD if BOOTCOUNT_LIMIT
 	imply TPL_CLK
 	imply TPL_DM