irqchip/vic: Cut down the external API

There are registers and functions in the header file
that are only used inside the driver. Move these into
the driver.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200607215124.48638-2-linus.walleij@linaro.org