dts: xmc4700_F144x2048: Merge dsram regions and use it as RAM

This is in preparation for xmc4xxx mdio/ethernet patch set. In the
ethernet drivers, the DMA memory (including descriptor and buffers)
must live in dsram1 or dsram2.

Currently, in xmc47_relax_kit the RAM is the psram1 region meaning
that DMA transfers will not work. Switch to using dsram regions instead.
Also, merge dsram1 and dsram2 into a single region since they are
contiguous.

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>