ARM: config: add DEVTMPFS option by default to keystone config

When using network driver on keystone and nfs rootfs is used following
error displayed and device halts.

Missing devtmpfs, which is required for udev to run
Halting...

Add the DEVTMPFS option by default to fix this
Signed-off-by: NMurali Karicheri <m-karicheri2@ti.com>
Signed-off-by: NSantosh Shilimkar <ssantosh@kernel.org>

arch/arm/configs/keystone_defconfig

@@ -201,3 +201,4 @@ CONFIG_KEYSTONE_IRQ=y
 CONFIG_GPIO_SYSCON=y
 CONFIG_TI_DAVINCI_MDIO=y
 CONFIG_MARVELL_PHY=y
+CONFIG_DEVTMPFS=y