dts: Renamed NXP usdhc in imxrt6xx

The names of these peripherals in the device tree
did not match the Reference Manual for the RT600.

Added alias for usdhc0 to be able to use SDHC test.

Also fixed a typo in a comment referring to USDHC which should have been
about USB.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>