bgmac: add bgmac_umac_*() helpers for accessing UniMAC registers

bgmac: add bgmac_umac_*() helpers for accessing UniMAC registers

UniMAC is a hardware block commonly used in Broadcom Ethernet controllers
that should get its own header file. Not every controller has it mapped at
the 0x800 offset so add bgmac access helpers. They will allow using
shared register defines.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20210107180051.1542-1-zajec5@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>