imx: ventana: enable pwm device-tree property based on hwconfig

imx: ventana: enable pwm device-tree property based on hwconfig

Most Ventana boards have a connector with off-board digital-I/O signals
including some that can be pinmuxed as either a PWM or a GPIO. The hwconfig
env variable is used to configure these and they will be pinmuxed according
to this configuration in the bootloader.

This patch adds a device-tree fixup that will enable the pwm controller
nodes appropriately for digital-I/O's that are configured as pwm via hwconfig
so that the pin can be used with the Linux kernel /sys/class/pwm API.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>