ARM: imx_v4_v5_defconfig: enable PWM driver

While there is no mainline board that makes use of the PWM still enable the
driver for it to increase compile test coverage.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>