arm64: dts: imx8mq: Fix boot from eMMC

The boot from eMMC is currently broken on the NXP i.MX8MQ EVK board.
When trying to boot from eMMC it fails with:

...
[    1.271938] mmc1: Tuning failed, falling back to fixed sampling clock
[    1.287429] print_req_error: I/O error, dev mmcblk1, sector 1 flags 0
[    1.306833] mmc1: Tuning failed, falling back to fixed sampling clock
[    1.322325] print_req_error: I/O error, dev mmcblk1, sector 2 flags 0
[    1.329559] Buffer I/O error on dev mmcblk1, logical block 0, async page read
[    1.336714]  mmcblk1: unable to read partition table
...

The problem is the result of a partial misconfiguration of the pins and
the missing assigned clock rate.

Fixes: 9079aca4

 ("arm64: add support for i.MX8M EVK board")
Signed-off-by: Carlo Caione <ccaione@baylibre.com>
Tested-by: Chris Spencer <christopher.spencer@sea.co.uk>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>