ipmi: kcs_bmc: Allow clients to control KCS IRQ state

Add a mechanism for controlling whether the client associated with a
KCS device will receive Input Buffer Full (IBF) and Output Buffer Empty
(OBE) events. This enables an abstract implementation of poll() for KCS
devices.

A wart in the implementation is that the ASPEED KCS devices don't
support an OBE interrupt for the BMC. Instead we pretend it has one by
polling the status register waiting for the Output Buffer Full (OBF) bit
to clear, and generating an event when OBE is observed.

Cc: CS20 KWLiu <KWLIU@nuvoton.com>
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Zev Weiss <zweiss@equinix.com>
Message-Id: <20210608104757.582199-10-andrew@aj.id.au>
Signed-off-by: Corey Minyard <cminyard@mvista.com>

ipmi: kcs_bmc: Strip private client data from struct kcs_bmc

Move all client-private data out of `struct kcs_bmc` into the KCS client
implementation.

With this change the KCS BMC core code now only concerns itself with
abstract `struct kcs_bmc` and `struct kcs_bmc_client` types, achieving
expected separation of concerns. Further, the change clears the path for
implementation of alternative userspace interfaces.

The chardev data-structures are rearranged in the same manner applied to
the KCS device driver data-structures in an earlier patch - `struct
kcs_bmc_client` is embedded in the client's private data and we exploit
container_of() to translate as required.

Finally, now that it is free of client data, `struct kcs_bmc` is renamed
to `struct kcs_bmc_device` to contrast `struct kcs_bmc_client`.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Zev Weiss <zweiss@equinix.com>
Message-Id: <20210608104757.582199-8-andrew@aj.id.au>
Signed-off-by: Corey Minyard <cminyard@mvista.com>

ipmi: kcs_bmc: Split headers into device and client

Strengthen the distinction between code that abstracts the
implementation of the KCS behaviours (device drivers) and code that
exploits KCS behaviours (clients). Neither needs to know about the APIs
required by the other, so provide separate headers.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Message-Id: <20210608104757.582199-7-andrew@aj.id.au>
Reviewed-by: Zev Weiss <zweiss@equinix.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>