x86: pci: Allow conditionally run VGA rom in S3

Introduce a new CONFIG_S3_VGA_ROM_RUN option so that U-Boot can
bypass executing VGA roms in S3.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Stefan Roese <sr@denx.de>