x86: crownbay: Adjust VGA rom address

binman complains when binary blobs are present:

  Node '/binman/rom/intel-vga': Offset 0xfff90000 (4294508544) overlaps
  with previous entry '/binman/rom/u-boot-dtb-with-ucode' ending at
  0xfff9204c (4294516812)

Adjust VGA rom address to 0xfffa0000 so that u-boot.rom image can be
successfully built again.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>