net: nixge: simplify code with devm platform functions

Use devm_platform_get_and_ioremap_resource() and
devm_platform_ioremap_resource_byname to simplify
code.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>