net: macb: Add shutdown operation support

Implement the shutdown hook to ensure clean and complete deactivation of
MACB controller. The shutdown sequence is protected with 'rtnl_lock()'
to serialize access and prevent race conditions while detaching and
closing the network device. This ensure a safe transition when the Kexec
utility calls the shutdown hook, facilitating seamless loading and
booting of a new kernel from the currently running one.

Signed-off-by: Abin Joseph <abin.joseph@amd.com>
Link: https://patch.msgid.link/20250610114111.1708614-1-abin.joseph@amd.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>