keystone2: keystone_nav: don't use hard addresses in netcp_pktdma

Use definitions in netcp_pktdma instead direct addresses.
The definitions can be set specifically for SoC, so there
is no reason to check SoC type while initialization.

Acked-by: Vitaly Andrianov <vitalya@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>