ARM: dts: i.MX6QDL: icore-rqs: Fix eMMC detection during SPL

usdhc4 has eMMC on icore-rqs boards, SPL is not detecting
it becuase of u-boot,dm-spl flag so add it to make eMMC working.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>