serial: txx9: Add missing #include <asm/txx9/generic.h>

drivers/tty/serial/serial_txx9.c:933:12: error: no previous prototype for ‘early_serial_txx9_setup’ [-Werror=missing-prototypes]
  933 | int __init early_serial_txx9_setup(struct uart_port *port)
      |            ^~~~~~~~~~~~~~~~~~~~~~~

This function is called from arch/mips/txx9/generic/setup.c, and does
have a forward declaration in arch/mips/include/asm/txx9/generic.h.

As the TXX9 serial driver does not support compile-testing, and thus can
only be built on MIPS, fix this by including the MIPS-only header file.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/818be2380061c19fe65819f7b7f10ab6e7aaa082.1706040343.git.geert+renesas@glider.beSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_txx9.c

@@ -23,9 +23,10 @@
 #include <linux/serial.h>
 #include <linux/tty.h>
 #include <linux/tty_flip.h>
-
 #include <linux/io.h>
 
+#include <asm/txx9/generic.h>
+
 #define PASS_LIMIT	256
 
 #if !defined(CONFIG_SERIAL_TXX9_STDSERIAL)