rpi: set the correct parameter for simple framebuffer node

When raspberrpi-4b platform  boots up, there are 2 sets of same bootup
log displayed on HDMI monitor screen, it looks like the screen is split
into 2 parts. The root cause is that video format of u-boot is different
from kernel. The fixing "a8r8g8b8" video format is used in u-boot, but
"r5g6b5" video format from framebuffer node is used in kernel image. In
order to avoid weird display status on screen, it needs to set the correct
parameter for simple framebuffer node even if it has existed.

Signed-off-by: Meng Li <Meng.Li@windriver.com>