device/pci_rom: Write _ROM method for VGA devices

Write _ROM method and store PCI Option ROM in CBMEM.

Allows an EFI compatible OS to retrieve the Option ROM without the need
to access the PCI BAR. As the Option ROM is no longer present in the
legacy VGA area it's required for mobile platforms. On hybrid devices,
like Lenovo Thinkpads supporting NVIDIA Optimus it's the only way to
retrieve the Option ROM, even with legacy BIOS, as there's no PCI BAR to
map.

Tested on:
* Lenovo T530
* Linux Kernel 4.13.7
* nouveau

Change-Id: I548b730fb64833083cc05af5b21dd6959804224b
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/20548
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/include/device/pci_rom.h

@@ -43,6 +43,8 @@ pci_rom_write_acpi_tables(struct device *device,
 						  unsigned long current,
 						  struct acpi_rsdp *rsdp);
 
+void pci_rom_ssdt(struct device *device);
+
 u32 map_oprom_vendev(u32 vendev);
 
 #endif