net: ll_temac: Support indirect_mutex share within TEMAC IP

Indirect register access goes through a DCR bus bridge, which
allows only one outstanding transaction.  And to make matters
worse, each TEMAC IP block contains two Ethernet interfaces, and
although they seem to have separate registers for indirect access,
they actually share the registers.  Or to be more specific, MSW, LSW
and CTL registers are physically shared between Ethernet interfaces
in same TEMAC IP, with RDY register being (almost) specificic to
the Ethernet interface.  The 0x10000 bit in RDY reflects combined
bus ready state though.

So we need to take care to synchronize not only within a single
device, but also between devices in same TEMAC IP.

This commit allows to do that with legacy platform devices.

For OF devices, the xlnx,compound parent of the temac node should be
used to find siblings, and setup a shared indirect_mutex between them.
I will leave this work to somebody else, as I don't have hardware to
test that.  No regression is introduced by that, as before this commit
using two Ethernet interfaces in same TEMAC block is simply broken.

Signed-off-by: Esben Haabendal <esben@geanix.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>