ddr: Migrate DDR_SPD to Kconfig

Move the symbol that controls building some JEDEC SPD support functions
to Kconfig.  This is required on the TI keystone 2 platforms and very
frequently (but not always) used on large number of Freescale/NXP
platforms, so use imply there.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 30fb7b6..c97acca 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -729,6 +729,7 @@
 	bool "TI Keystone"
 	select CMD_POWEROFF
 	select CPU_V7A
+	select DDR_SPD
 	select GPIO_EXTRA_HEADER
 	select SUPPORT_SPL
 	select SYS_ARCH_TIMER