spl: dm: Add SPL_DM_SEQ_ALIAS config option

The Device Model sequence alias feature is required by some Uclasses.
Instead of disabling the feature for all SPL targets allow it to be
configured.

The config option is disabled by default to reduce code size for targets
that are not interested or do not require this feature.

Signed-off-by: Nathan Rossi <nathan@nathanrossi.com>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Marek Vasut <marex@denx.de>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: Stefan Roese <sr@denx.de>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>