ar7: fix kernel builds for compiler test

TI AR7 Watchdog Timer is only build for 32bit.

Avoid error like:
In file included from drivers/watchdog/ar7_wdt.c:29:
./arch/mips/include/asm/mach-ar7/ar7.h: In function ‘ar7_is_titan’:
./arch/mips/include/asm/mach-ar7/ar7.h:111:24: error: implicit declaration of function ‘KSEG1ADDR’; did you mean ‘CKSEG1ADDR’? [-Werror=implicit-function-declaration]
  111 |  return (readl((void *)KSEG1ADDR(AR7_REGS_GPIO + 0x24)) & 0xffff) ==
      |                        ^~~~~~~~~
      |                        CKSEG1ADDR

Fixes: da2a68b3 ("watchdog: Enable COMPILE_TEST where possible")
Signed-off-by: Jackie Liu <liuyun01@kylinos.cn>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20210907024904.4127611-1-liu.yun@linux.devSigned-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>

drivers/watchdog/Kconfig

@@ -1664,7 +1664,7 @@ config SIBYTE_WDOG
 
 config AR7_WDT
 	tristate "TI AR7 Watchdog Timer"
-	depends on AR7 || (MIPS && COMPILE_TEST)
+	depends on AR7 || (MIPS && 32BIT && COMPILE_TEST)
 	help
 	  Hardware driver for the TI AR7 Watchdog Timer.