MIPS: alchemy: xxs1500: add gpio-au1000.h header file

board-xxs1500.c references 2 functions without declaring them, so add
the header file to placate the build.

../arch/mips/alchemy/board-xxs1500.c: In function 'board_setup':
../arch/mips/alchemy/board-xxs1500.c:56:2: error: implicit declaration of function 'alchemy_gpio1_input_enable' [-Werror=implicit-function-declaration]
   56 |  alchemy_gpio1_input_enable();
../arch/mips/alchemy/board-xxs1500.c:57:2: error: implicit declaration of function 'alchemy_gpio2_enable'; did you mean 'alchemy_uart_enable'? [-Werror=implicit-function-declaration]
   57 |  alchemy_gpio2_enable();

Fixes: 8e026910 ("MIPS: Alchemy: merge GPR/MTX-1/XXS1500 board code into single files")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: linux-mips@vger.kernel.org
Cc: Manuel Lauss <manuel.lauss@googlemail.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Acked-by: Manuel Lauss <manuel.lauss@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>