net: hibmcge: Add read/write registers supported through the bar space

Add support for to read and write registers through the pic bar space.

Some driver parameters, such as mac_id, are determined by the
board form. Therefore, these parameters are initialized
from the register as device specifications.

the device specifications register are initialized and written by bmc.
driver will read these registers when loading.

Signed-off-by: Jijie Shao <shaojijie@huawei.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>