This change adds `BOOT_DEVICE_NVME` to the `enum` list in
`arch/x86/include/asm/spl.h`,
enabling NVMe as a recognized boot device for SPL (Secondary Program
Loader).
Tested x86 hardware with coreboot + U-Boot payload.
Verified successful boot to NVMe drive.
Signed-off-by: Naresh Solanki <naresh.solanki@9elements.com> Reviewed-by: Simon Glass <sjg@chromium.org>