ARM: imx: update the cpu power up timing setting on i.mx6sx

The sw2iso count should cover ARM LDO ramp-up time,
the MAX ARM LDO ramp-up time may be up to more than
100us on some boards, this patch sets sw2iso to 0xf
(~384us) which is the reset value, and it is much
more safe to cover different boards, since we have
observed that some customer boards failed with current
setting of 0x2.

Fixes: 05136f08 ("ARM: imx: support arm power off in cpuidle for i.mx6sx")
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>