x86: spl: Add support for NVMe boot device

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>

arch/x86/include/asm/spl.h

@@ -11,6 +11,7 @@ enum {
 	BOOT_DEVICE_SPI_MMAP	= 10,
 	BOOT_DEVICE_FAST_SPI,
 	BOOT_DEVICE_CROS_VBOOT,
+	BOOT_DEVICE_NVME,
 };
 
 void jump_to_spl(ulong entry);