ARM: dts: imx7: add ftm nodes for Flex Timers

The i.MX7 has two possible Flex Timers, disabled by default. Moreover, the
block is the same as LS1021a, then the drivers can be used as-is.

Signed-off-by: Thomas Perrot <thomas.perrot@bootlin.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>