net: phy: cavium: Improve __iomem mess

The MIPS low level register access functions seem to be missing
__iomem annotation. This causes lots of sparse warnings, when code
casts off the __iomem. Make the Cavium MDIO drivers cleaner by pushing
the casts lower down into the helpers, allow the drivers to work as
normal, with __iomem.

bus->register_base is now an void *, rather than a u64. So forming the
mii_bus->id string cannot use %llx any more. Use %px, so this kernel
address is still exposed to user space, as it was before.

v2: s/cases/causes/g

Cc: Sunil Goutham <sgoutham@marvell.com>
Cc: Robert Richter <rrichter@marvell.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>