ARM: imx: add timer stop flag to ARM power off state

When the CPU is in ARM power off state the ARM architected
timers are stopped. The flag is already present in the higher
power WAIT mode.

This allows to use the ARM generic timer on i.MX 6UL/6ULL SoC.
Without the flag the kernel freezes when the timer enters the
first time ARM power off mode.

Note: The default timer on i.MX6SX is the i.MX GPT timer which is
not disabled during CPU idle. However, the timer is not affected
by the CPUIDLE_FLAG_TIMER_STOP flag. The flag only affects CPU
local timers.

Cc: Anson Huang <anson.huang@nxp.com>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>