fb: arcpgu: Add switch to select between RGB888/RGB565

AXS101 needs RGB565 for optimal performance, while nSIM OSCI
needs RGB888 for optimal compatibility.

Signed-off-by: Mischa Jonker <mjonker@synopsys.com>
ARC: [nsimosci] Update defconfig

This updates the defconfig to select the proper display mode in the
ARCPGU framebuffer driver. This is required now, since the ARCPGU
driver has been extended to work with the AXS101 platform.

Signed-off-by: Mischa Jonker <mjonker@synopsys.com>