tty/powerpc: early udbg consoles can't be modules

Fixes these build errors:

ERROR: ".udbg_printf" [drivers/tty/ehv_bytechan.ko] undefined!
ERROR: ".register_early_udbg_console" [drivers/tty/ehv_bytechan.ko] undefined!
ERROR: "udbg_putc" [drivers/tty/ehv_bytechan.ko] undefined!

Cc: Timur Tabi <timur@freescale.com>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>