net: ti: am65-cpsw: Cleanup resources before jump to kernel

In case fastboot over Ethernet, am65_cpsw_stop() is not called unless
DM_FLAG_OS_PREPARE is set. Without call to am65_cpsw_stop(), DMA
resources are not released thus leading to failures in kernel.
Fix this by adding DM_FLAG_OS_PREPARE flag to am65_cpsw_nuss_port
driver.

Reported-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>