roms/Makefile: fix npcmNxx_bootrom build rules

roms/Makefile: fix npcmNxx_bootrom build rules

Since commit 70ce076fa6dff60, the actual rom source dirs
are subdirs of vbootrom/ submodule, not in top-level of it.

Fixes: 70ce076fa6dff60 "roms: Update vbootrom to 1287b6e"
Fixes: 269b7effd90 ("pc-bios: Add NPCM8XX vBootrom")
Cc: qemu-stable@nongnu.org
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250727215511.807880-1-mjt@tls.msk.ru>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>