ARM: imx_v6_v7_defconfig: Set THERMAL_WRITABLE_TRIPS=y for testing

Setting trip points is supported by the imx thermal driver and it is
useful to be able to test this without adjusting config.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>