MAINTAINERS: add myself as maintainer for spi-fsl-dspi driver

Since I've introduced a fairly large diff to this driver since tag v5.4,
I would like to avoid breakage for my use cases by getting a chance to
be copied on newly submitted patches.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20200821213753.3143632-1-olteanv@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>