arm: spear600: Add missing interrupt-parent of rtc

The interrupt-parent of rtc was missing, add it.

Fixes: 8113ba91 ("ARM: SPEAr: DT: Update device nodes")
Cc: stable@vger.kernel.org # v3.8+
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>