[IMX6][VIDEO] fix video configuration and splashscreen
- CONFIG_VIDEO is incompatible with CONFIG_DM_VIDEO (needed for IMX IPUv3 graphic)
- CONFIG_SPLASH_SCREEN, CONFIG_SPLASH_SCREEN_ALIGN moved to defconfig
- they are defined in a Kconfig, so is better to put them in the defconfig
- CONFIG_SPLASH_SCREEN is checked in file common/stdio.c with the macro IS_ENABLED(), so it has to be defined in the defconfig in order to be correctly checked
- CONFIG_HIDE_LOGO_VERSION used to hide the version string on the display
- add default SPLASHIMAGE_ADDR value
- set centered splash as default position