watchdog: db8500: add back prmcu dependency

When the db8500 watchdog is enabled without the PRCMU, we get a lot of
warnings about duplicate or missing helper functions:

In file included from drivers/watchdog/ux500_wdt.c:21:0:
include/linux/mfd/dbx500-prcmu.h:422:19: error: redefinition of 'prcmu_abb_read'
 static inline int prcmu_abb_read(u8 slave, u8 reg, u8 *value, u8 size)

This restores the dependency as it was.

Fixes: da2a68b3 ("watchdog: Enable COMPILE_TEST where possible")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>