powerpc: rtas_flash needs to use rtas_data_buf

When trying to flash a machine via the update_flash command, Anton received the
following error:

    Restarting system.
    FLASH: kernel bug...flash list header addr above 4GB

The code in question has a comment that the flash list should be in
the kernel data and therefore under 4GB:

        /* NOTE: the "first" block list is a global var with no data
         * blocks in the kernel data segment.  We do this because
         * we want to ensure this block_list addr is under 4GB.
         */

Unfortunately the Kconfig option is marked tristate which means the variable
may not be in the kernel data and could be above 4GB.

Instead of relying on the data segment being below 4GB, use the static
data buffer allocated by the kernel for use by rtas.  Since we don't
use the header struct directly anymore, convert it to a simple pointer.

Reported-By: Anton Blanchard <anton@samba.org>
Signed-Off-By: Milton Miller <miltonm@bga.com>
Tested-By: Anton Blanchard <anton@samba.org>

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>