ARM: dts: imx6ull: change to use new compatible "fsl,imx6ull-usdhc" for usdhc

i.MX6ULL has errata ERR010450, there is I/O timing limitation,
for SDR mode, SD card clock can't exceed 150MHz, for DDR mode,
SD card clock can't exceed 45MHz. This patch change to use the
new compatible "fsl,imx6ull-usdhc" to follow this limitation.

Signed-off-by: Haibo Chen <haibo.chen@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>