Add board level codes for enabling splash screen on imx8mm EVK. We
support two different display connecting to MIPI DSI miniSAS interfaces:

1. MIPI2HDMI daughter card (default)
2. RM67191 OLED panel

Users can set "panel" env vairable to "MIPI2HDMI" or "RM67191_OLED" to
switch them after reboot.

Signed-off-by: Ye Li <ye.li@nxp.com>