net: dsa: mv88e6xxx: update code operating on hidden registers

This patch moves the functions operating on the hidden debug registers
into it's own file, port_hidden.c. The functions prefix is renamed from
mv88e6390_hidden_ to mv88e6xxx_port_hidden_, to be consistent with the
rest of this driver. The macros are prefixed with MV88E6XXX_ prefix, and
are changed not to use the BIT() macro nor bit shifts, since the rest of
the port.h file does not use it.

We also add the support for setting the Block Address field when
operating hidden registers. Marvell's mdio examples for SERDES settings
on Topaz use Block Address 0x7 when reading/writing hidden registers,
and although the specification says that block must be set to 0xf, those
settings are reachable only with Block Address 0x7.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>