rockchip: enable DEBUG_UART_BOARD_INIT by default

All Rockchip SoCs use DEBUG_UART_BOARD_INIT to init per board
UART IOMUX, enable it by default.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 0a76138..50daec7 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1439,6 +1439,7 @@
 	select SYS_THUMB_BUILD if !ARM64
 	imply ADC
 	imply CMD_DM
+	imply DEBUG_UART_BOARD_INIT
 	imply DISTRO_DEFAULTS
 	imply FAT_WRITE
 	imply SARADC_ROCKCHIP