ARM: imx_v6_v7_defconfig: Explicitly restore CONFIG_DEBUG_FS

This option was removed by "make savedefconfig" in
commit c5054a98 ("ARM: imx_v6_v7_defconfig: Select SMSC_PHY")

This happened because CONFIG_DEBUG_FS was implicitly selected by
CONFIG_TREE_RCU_TRACE which defaulted to true because CONFIG_RCU_TRACE
was enabled by default by commit 96151825 ("rcu: Enable RCU
tracepoints by default to aid in debugging")

Recently however CONFIG_RCU_TRACE was completely removed by
commit 6e74c237c410 ("rcu: Remove debugfs tracing")

The result is that imx_v6_v7_defconfig no longer includes DEBUG_FS on
linux-next since next-20170517. This is bad, DEBUG_FS is extremely
useful for kernel introspection and testing.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>