net: mscc: ocelot: introduce a new ocelot_target_{read,write} API

There are some targets (register blocks) in the Ocelot switch that are
instantiated more than once. For example, the VCAP IS1, IS2 and ES0
blocks all share the same register layout for interacting with the cache
for the TCAM and the action RAM.

For the VCAPs, the procedure for servicing them is actually common. We
just need an API specifying which VCAP we are talking to, and we do that
via these raw ocelot_target_read and ocelot_target_write accessors.

In plain ocelot_read, the target is encoded into the register enum
itself:

	u16 target = reg >> TARGET_OFFSET;

For the VCAPs, the registers are currently defined like this:

	enum ocelot_reg {
	[...]
		S2_CORE_UPDATE_CTRL = S2 << TARGET_OFFSET,
		S2_CORE_MV_CFG,
		S2_CACHE_ENTRY_DAT,
		S2_CACHE_MASK_DAT,
		S2_CACHE_ACTION_DAT,
		S2_CACHE_CNT_DAT,
		S2_CACHE_TG_DAT,
	[...]
	};

which is precisely what we want to avoid, because we'd have to duplicate
the same register map for S1 and for S0, and then figure out how to pass
VCAP instance-specific registers to the ocelot_read calls (basically
another lookup table that undoes the effect of shifting with
TARGET_OFFSET).

So for some targets, propose a more raw API, similar to what is
currently done with ocelot_port_readl and ocelot_port_writel. Those
targets can only be accessed with ocelot_target_{read,write} and not
with ocelot_{read,write} after the conversion, which is fine.

The VCAP registers are not actually modified to use this new API as of
this patch. They will be modified in the next one.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>