board: stm32mp1: add splash screen with stmicroelectronics logo

Display the STMicroelectronics logo with features VIDEO_LOGO and
SPLASH_SCREEN on STMicroelectronics boards.

With CONFIG_SYS_VENDOR = "st", the logo st.bmp is selected, loaded at the
address indicated by splashimage and centered with "splashpos=m,m".

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>