ipmi: kcs_bmc: Split out kcs_bmc_cdev_ipmi

Take steps towards defining a coherent API to separate the KCS device
drivers from the userspace interface. Decreasing the coupling will
improve the separation of concerns and enable the introduction of
alternative userspace interfaces.

For now, simply split the chardev logic out to a separate file. The code
continues to build into the same module.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Zev Weiss <zweiss@equinix.com>
Message-Id: <20210608104757.582199-5-andrew@aj.id.au>
Signed-off-by: Corey Minyard <cminyard@mvista.com>