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/mips/Kconfig b/arch/mips/Kconfig
index 569f5f4..f0704d9 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -77,6 +77,7 @@
 	select PINMUX
 	select PINCONF
 	select RESET_MTMIPS
+	imply MTD
 	imply DM_SPI
 	imply DM_SPI_FLASH
 	select LAST_STAGE_INIT
@@ -109,6 +110,7 @@
 	select MIPS_L2_CACHE
 	select MIPS_MACH_EARLY_INIT
 	select MIPS_TUNE_OCTEON3
+	select MTD
 	select ROM_EXCEPTION_VECTORS
 	select SUPPORTS_BIG_ENDIAN
 	select SUPPORTS_CPU_MIPS64_OCTEON