mtd: Make CONFIG_MTD be the gate symbol for the menu

The help for CONFIG_MTD explains that it needs to be enabled for various
things like NAND, etc to be available. It however then doesn't enforce
this dependency and so if you have none of these systems present you
still need to disable a number of options. Fix this by making places
that select/imply one type of flash, but did not do the same, also do
this for "MTD". Make boards which hadn't been enabling MTD already but
need it now, do so. In a few places, disable CONFIG_CMD_MTDPARTS as it
wasn't previously enabled but was now being implied.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/arm/mach-imx/mx6/Kconfig b/arch/arm/mach-imx/mx6/Kconfig
index 50a9c3e..114cce4 100644
--- a/arch/arm/mach-imx/mx6/Kconfig
+++ b/arch/arm/mach-imx/mx6/Kconfig
@@ -201,6 +201,7 @@
 	select DM_SPI
 	select DM_GPIO
 	select DM_SERIAL
+	select MTD
 	select SUPPORT_SPL
 	imply CMD_DM
 
@@ -261,6 +262,7 @@
 	select DM_SERIAL
 	select DM_I2C
 	select DM_SPI
+	select MTD
 	imply CMD_DM
 
 config TARGET_MX6CUBOXI
@@ -602,6 +604,7 @@
 	imply DM
 	imply DM_GPIO
 	imply DM_MMC
+	imply MTD
 	imply DM_SPI
 	imply DM_SPI_FLASH
 	imply DM_I2C