ARM: tegra: apalis-tk1: Activate PWM pin muxing for pwm3

Activate PWM pin muxing for Apalis PWM3. Note that the same PWM3 is
already active on pu6 being Apalis BKL1_PWM as well. Therefore exporting
that one for raw sysfs access will fail and one has to revert to using
the PWM backlight.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>