cmd: ubi: change 'default y' for SUNXI to 'imply' in Kconfig

It is not preferred to put SUNXI-specific code in the common place.

Change it to 'imply' property of ARCH_SUNXI.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 9f5eaf8..8a23c76 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -831,6 +831,7 @@
 	select USE_TINY_PRINTF
 	imply CMD_DM
 	imply CMD_GPT
+	imply CMD_UBI if NAND
 	imply DISTRO_DEFAULTS
 	imply FAT_WRITE
 	imply OF_LIBFDT_OVERLAY