ARM: dts: imx6q-ba16: Disable pwm2 by default

pwm2 is provided on the BA16 Q7 module, but is not used on any
of the current configurations. However, future platforms may
utilize this device, so we are simply disabling the node rather
than removing it completely.

Signed-off-by: Justin Waters <justin.waters@timeys.com>
Signed-off-by: Akshay Bhat <akshay.bhat@timesys.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>