imx6ul-14x14-evk does not have a GPIO dedicated for reading the card
detect pin on the eSDHC2 micro-SD port.

Pass the "broken-cd" property to describe the absence of the card detect
GPIO so that polling must be used.

According to Documentation/devicetree/bindings/mmc/mmc-controller.yaml:

  broken-cd:
    $ref: /schemas/types.yaml#/definitions/flag
    description:
      There is no card detection available; polling must be used.

Even though no error is oberved in the kernel, the lack of the
'broken-cd' property caused the micro-SD to not be detected in U-Boot,
so let's improve the device tree description to make it more accurate.
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>