soc/integration: Ensure the video framebuffer fits in main RAM

[davidar]

On targets with less than 20MiB of SDRAM, the default framebuffer region overflows the available address space, leading to errors like this:

litex> mem_list
Available memory regions:
ROM                0x00000000 0x20000 
SRAM               0x10000000 0x2000 
SPIFLASH           0x00800000 0x800000 
MAIN_RAM           0x40000000 0x800000 
VIDEO_FRAMEBUFFER  0x40c00000 0x800000 
CSR                0xf0000000 0x10000 

litex> mem_test 0x40000000 0x800000
Memtest at 0x40000000 (8.0MiB)...
  Write: 0x40000000-0x40800000 8.0MiB     
   Read: 0x40000000-0x40800000 8.0MiB     
Memtest OK

litex> mem_test 0x40c00000 0x800000
Memtest at 0x40c00000 (8.0MiB)...
memtest_access error @ 0x40c00000, exiting memtest.

This change resizes the framebuffer to occupy no more than half the available space and relocates it to before the end of the main_ram region on such targets.

[enjoy-digital]

Thanks @davidar, this make sense. I'll review it very soon.