net: ldpaa_eth: transform dpni_statistics from a struct to a union

In order to simplify code, dpni_statistics can be written as a union.
Using the raw accessors we can just loop through all the statistics from
a page without trying to access each an every one independently.
Make this change to a union.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>