serial: make VPL_DM_SERIAL depend on VPL_DM

serial: make VPL_DM_SERIAL depend on VPL_DM

I have a hunch VPL_DM_SERIAL should not be selectable if VPL isn't set
as implied by the prefix. Additionally, still based on the prefix, I'm
assuming VPL_DM should be a dependency. Since VPL_DM can only be
selectable when VPL is enabled, only depend on VPL_DM.

This mirrors SPL_DM_SERIAL and TPL_DM_SERIAL so seems right to me.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>