dt: remove extra xsysace platform_driver registration

After commit 1c48a5c93, "dt: Eliminate
of_platform_{,un}register_driver", the xsysace driver attempts to
register two platform_drivers with the same name, which a) doesn't
work, and b) isn't necessary.  This patch merges the two
platform_drivers.

Reported-by: Daniel Hellstrom <daniel@gaisler.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>