drivers: ipmi: Support raw i2c packet in IPMB

Many IPMB devices don't support smbus protocol and this driver
only supports the smbus protocol at the moment.

Added support for the i2c protocol as well. There will be a variable
"i2c-protocol" passed by the device tree or ACPI table which determines
whether the protocol is i2c or smbus.

Signed-off-by: Vijay Khemka <vijaykhemka@fb.com>
Reviewed-by: Asmaa Mnebhi <asmaa@mellanox.com>
Message-Id: <20191211185604.1266063-1-vijaykhemka@fb.com>
[IPMB.txt had moved to driver-api/ipmb.rst, I adjusted]
Signed-off-by: Corey Minyard <cminyard@mvista.com>