pwm: fsl-ftm: Document 'big-endian' property

The same FTM PWM device can have a different endianness on different
SoCs. The device tree provides a property to describing this so that an
operating system device driver can handle all variants of the device.
Refer to the table below for the endianness of the FTM PWM block as
integrated into the existing SoCs:

	SoC     | FTM-PWM endianness
	--------+-------------------
	Vybrid  | LE
	LS1     | BE
	LS2     | LE

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>