net: bgmac: Correctly annotate register space

All the members: base, idm_base and nicpm_base should be annotated with
__iomem since they are pointers to register space. This fixes a bunch of
sparse reported warnings.

Fixes: f6a95a24 ("net: ethernet: bgmac: Add platform device support")
Fixes: dd5c5d03 ("net: ethernet: bgmac: add NS2 support")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>