arm: mvebu: spl: Use IS_ENABLED() instead of #ifdef where possible

Use the preferred
  if (IS_ENABLED(X))
instead of
  #ifdef X
where possible.

There are still places where this is not possible or is more complicated
to convert in this file. Leave those be for now.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>