UBUNTU: [Config] Enable CONFIG_DEBUG_KERNEL for ARM branch

BugLink: http://bugs.launchpad.net/bugs/513241

We need to enable CONFIG_TIMER_STATS to use powertop properly on ARM hardware.
But CONFIG_TIMER_STATS depends on CONFIG_DEBUG_KERNEL. So we turn on this
option here as well as x86 branches.

This patch can share with all ARM branches

Signed-off-by: Bryan Wu <bryan.wu@canonical.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>