boards: arm: stm32f429i_disc1: update onboard display

Onboard display is actually an ILI9341. It is connected to SPI5 which
is shared with L3GD20. Max clock is set as described in BSP driver files
(stm32f429i_discovery.c@638). ILI9341 configuration updated to enable
RGB interface use.

Signed-off-by: Konstantinos Papadopoulos <kostas.papadopulos@gmail.com>