imx8qxp: mek: default enable SPL

Enable SPL for i.MX8QXP MEK, and currently use SPL FIT.
The SPL enable SPL_DM to use MMC/PINCTRL/POWER DOMAIN/CLK.

Note: SPL FIT could not support secure boot chain, because i.MX8/8X
only support i.MX container format. This container format has
not been upstreamed, so we use FIT for now. When SPL container
supported, we could switch to that.

Signed-off-by: Peng Fan <peng.fan@nxp.com>