ARM: imx_v6_v7_defconfig: continue compiling the pwm driver

After the pwm-imx driver was split into two drivers and the Kconfig symbol
changed accordingly, use the new name to continue being able to use the
PWM hardware.

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