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: 8e026910fcd4 ("MIPS: Alchemy: merge GPR/MTX-1/XXS1500 board code into single files")
Signed-off-by: Randy Dunlap <[e-mail]>
Cc: Thomas Bogendoerfer <[e-mail]>
Cc: [e-mail]
Cc: Manuel Lauss <[e-mail]>
Cc: Ralf Baechle <[e-mail]>
Acked-by: Manuel Lauss <[e-mail]>
Signed-off-by: Thomas Bogendoerfer <[e-mail]>

arch/mips/alchemy/board-xxs1500.c

···
 #include <asm/reboot.h>
 #include <asm/setup.h>
 #include <asm/mach-au1x00/au1000.h>
+#include <asm/mach-au1x00/gpio-au1000.h>
 #include <prom.h>
 
 const char *get_system_type(void)