microblaze: Enable options to mount a rootfs via NFS

Enable the options to mount a rootfs over NFS for microblaze
platforms and also support for automatic configuration of IP addresses
during boot as needed by NFS.

Signed-off-by: Raju Kumar Pothuraju <rajukumar.pothuraju@amd.com>
Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@amd.com>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/d7bc1c93e485e0a6ab2c234a144402b7bd5f77bb.1698155190.git.michal.simek@amd.com