dma: keystone_nav: move keystone_nav driver to driver/dma/

The keystone_nav is used by drivers/net/keystone_net.c driver to
send and receive packets, but currently it's placed at keystone
arch sources. So it should be in the drivers directory also.
It's separate driver that can be used for sending and receiving
pktdma packets by others drivers also.

This patch just move this driver to appropriate directory and
doesn't add any functional changes.

Acked-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>