MIPS: ralink: export rt_sysc_membase for rt2880_wdt.c

MIPS: ralink: export rt_sysc_membase for rt2880_wdt.c

rt2880_wdt.c uses (well, attempts to use) rt_sysc_membase. However,
when this watchdog driver is built as a loadable module, there is a
build error since the rt_sysc_membase symbol is not exported.
Export it to quell the build error.

ERROR: modpost: "rt_sysc_membase" [drivers/watchdog/rt2880_wdt.ko] undefined!

Fixes: 473cf939ff34 ("watchdog: add ralink watchdog driver")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Wim Van Sebroeck <wim@iguana.be>
Cc: John Crispin <john@phrozen.org>
Cc: linux-mips@vger.kernel.org
Cc: linux-watchdog@vger.kernel.org
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>