sh: hd64461: Handle virq offset for offchip IRQ base and HD64461 IRQ

sh: hd64461: Handle virq offset for offchip IRQ base and HD64461 IRQ

A recent change to start counting SuperH IRQ #s from 16 breaks support
for the Hitachi HD64461 companion chip.

Move the offchip IRQ base and HD64461 IRQ # by 16 in order to
accommodate for the new virq numbering rules.

Fixes: a8ac2961148e ("sh: Avoid using IRQ0 on SH3 and SH4")
Signed-off-by: Artur Rojek <contact@artur-rojek.eu>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/r/20230710233132.69734-1-contact@artur-rojek.eu
Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>