board: stm32f746-disco: refactor the display of the ST logo

The patch removes the legacy mode of displaying the ST logo and adopts
the approach introduced by the commit 284b08fb ("board: stm32mp1: add
splash screen with stmicroelectronics logo"). It was necessary to use a
specific logo for the stm32f746-disco board.

Furthermore, the previous version didn't properly center the logo, hiding
its upper part.

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