irqchip: VIC: export vic_init_cascaded

vic_init_cascaded is called by integrator impd1 code that can
be a loadable module, so the function has to be exported.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>