ipmi: add an Aspeed BT IPMI BMC driver

This patch adds a simple device driver to expose the iBT interface on
Aspeed SOCs (AST2400 and AST2500) as a character device. Such SOCs are
commonly used as BMCs (BaseBoard Management Controllers) and this
driver implements the BMC side of the BT interface.

The BT (Block Transfer) interface is used to perform in-band IPMI
communication between a host and its BMC. Entire messages are buffered
before sending a notification to the other end, host or BMC, that
there is data to be read. Usually, the host emits requests and the BMC
responses but the specification provides a mean for the BMC to send
SMS Attention (BMC-to-Host attention or System Management Software
attention) messages.

For this purpose, the driver introduces a specific ioctl on the
device: 'BT_BMC_IOCTL_SMS_ATN' that can be used by the system running
on the BMC to signal the host of such an event.

The device name defaults to '/dev/ipmi-bt-host'

Signed-off-by: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>
[clg: - checkpatch fixes
      - added a devicetree binding documentation
      - replace 'bt_host' by 'bt_bmc' to reflect that the driver is
        the BMC side of the IPMI BT interface
      - renamed the device to 'ipmi-bt-host'
      - introduced a temporary buffer to copy_{to,from}_user
      - used platform_get_irq()
      - moved the driver under drivers/char/ipmi/ but kept it as a misc
        device
      - changed the compatible cell to "aspeed,ast2400-bt-bmc"
]
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
[clg: - checkpatch --strict fixes
      - removed the use of devm_iounmap, devm_kfree in cleanup paths
      - introduced an atomic-t to limit opens to 1
      - introduced a mutex to protect write/read operations]
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Corey Minyard <cminyard@mvista.com>