MFC 278321:
Use direct hardware access for internal requests for KCS and SMIC. In
particular, updates to the watchdog should no longer sleep.

• Add a new IPMI_IO_LOCK for low-level I/O access. Use this for kcs_polled_request() and smic_polled_request().
• Add a new backend callback "ipmi_driver_request" to handle a driver request. The new callback performs the request sychronously for KCS and SMIC. SSIF still defers the work to the worker thread since the worker thread sleeps during request processing anyway.
• Allocate driver requests on the stack rather than using malloc().