net: ll_temac: Add support for non-native register endianness

Replace the powerpc specific MMIO register access functions with the
generic big-endian mmio access functions, and add support for
little-endian access depending on configuration.

Big-endian access is maintained as the default, but little-endian can
be configured in device-tree binding or in platform data.

The temac_ior()/temac_iow() functions are replaced with macro wrappers
to avoid modifying existing code more than necessary.

Signed-off-by: Esben Haabendal <esben@geanix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>