vpl: fix reference in comment to non-existing SPL_SERIAL_SUPPORT

Since commit 2a736066 ("serial: Rename SERIAL_SUPPORT to SERIAL")
SPL_SERIAL_SUPPORT is named SPL_SERIAL. So let's update the comment to
point to the correct Kconfig option in the comment of VPL_SERIAL.

Fixes: 747093dd ("vpl: Add Kconfig options for VPL")
Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>