dts: pinctrl: kinetis: make slew-rate optional

Setting slew-rate property is not supported on Kinetis KE series
and the value will not have effect, so this property should not
be required.

We are also planning to reuse the Kinetis pin control binding and
associated driver for NXP S32K1xx devices, which doesn't support
setting the slew-rate rate as well.

Signed-off-by: Manuel Argüelles <manuel.arguelles@nxp.com>