net: phy: mdio-bcm-unimac: Use correct I/O accessors

The driver currently uses __raw_{read,write}l which works for all
platforms supported: Broadcom MIPS LE/BE (native endian), ARM LE (native
endian) but not ARM BE (registers are still LE). Switch to using the
proper accessors for all platforms and explain why Broadcom MIPS BE is
special here, in doing so, we introduce a couple of helper functions to
abstract these differences.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>