powerpc/powernv: Add driver for operator panel on FSP machines

Implement new character device driver to allow access from user space
to the operator panel display present on IBM Power Systems machines
with FSPs.

This will allow status information to be presented on the display which
is visible to a user.

The driver implements a character buffer which a user can read/write
by accessing the device (/dev/op_panel). This buffer is then displayed on
the operator panel display. Any attempt to write past the last character
position will have no effect and attempts to write more characters than
the size of the display will be truncated. The device may only be accessed
by a single process at a time.

Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/opal.h

@@ -178,6 +178,8 @@ int64_t opal_dump_ack(uint32_t dump_id);
 int64_t opal_dump_resend_notification(void);
 
 int64_t opal_get_msg(uint64_t buffer, uint64_t size);
+int64_t opal_write_oppanel_async(uint64_t token, oppanel_line_t *lines,
+					uint64_t num_lines);
 int64_t opal_check_completion(uint64_t buffer, uint64_t size, uint64_t token);
 int64_t opal_sync_host_reboot(void);
 int64_t opal_get_param(uint64_t token, uint32_t param_id, uint64_t buffer,