imx: mx6quq7: fix USDHC4 declaration

USDHC2 does not exist on mx6quq7 board, as USDHC4 was already been
declared this is probably a typo.

Signed-off-by: Julien Corjon <corjon.j@ecagroup.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>