of/platform: Fix microblaze build failure

Commit bf5db2fb (microblaze: Use generic device.h) removes the
microblaze specific pdev_archdata and dma_mask.

At the same time, commit 591c1ee4 (of: configure the platform
device dma parameters) initializes the just removed field.
This causes all microblaze builds to fail.

Drop the unnecessary initialization.

Cc: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: Rob Herring <robh@kernel.org>