imx6ulpico: linux.fragment: Disable CONFIG_SMP

imx6ul has currently an issue on kernel 4.7 that causes a stall when running
the "reboot" command.

This issue has been reported in the linux-arm-kernel mailing list, but we
don't have a proper fix at the moment.

This problem is not seen when SMP is disabled, so let's disable it for now
until a proper fix becomes available.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>