dt-bindings: timer: fsl,ftm-timer: use items for reg

dt-bindings: timer: fsl,ftm-timer: use items for reg

The original txt binding doc is:
  reg : Specifies base physical address and size of the register sets for
        the clock event device and clock source device.

And existed dts provide two reg MMIO spaces. So change to use items to
descript reg property.

Update examples.

Fixes: 8fc30d8f8e86 ("dt-bindings: timer: fsl,ftm-timer: Convert to dtschema")
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250523141437.533643-1-Frank.Li@nxp.com