irqchip/vic: Drop cascaded intialization call

We got rid of the last user of the cascaded intialization
from board files so drop this API.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200607215124.48638-1-linus.walleij@linaro.org