ipmi: kcs: aspeed: Update port address comments

Remove AST_usrGuide_KCS.pdf as it is no longer maintained.

Add more descriptions as the driver now supports the I/O
address configurations for both the KCS Data and Cmd/Status
interface registers.

Signed-off-by: Chia-Wei Wang <chiawei_wang@aspeedtech.com>
Message-Id: <20220920020333.601-1-chiawei_wang@aspeedtech.com>
[I don't like removing documentation, but the document in question
 was a personal note by an employee and nothing official and not
 necessarily guaranteed to be accurate in the future.  So go
 ahead and remove it.]
Signed-off-by: Corey Minyard <cminyard@mvista.com>