net: ethernet: ti: Add accessors for struct k3_cppi_desc_pool members

This patch adds accessors for desc_size and cpumem members. They may be
used, for instance, to compute a descriptor index.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Julien Panis <jpanis@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>