dt-bindings: rtc: add bindings for FlexTimer Module

The patch adds bindings for FlexTimer Module

Signed-off-by: Biwen Li <biwen.li@nxp.com>
Link: https://lore.kernel.org/r/20190813030157.48590-2-biwen.li@nxp.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
