dt-bindings: Drop required 'interrupt-parent'

'interrupt-parent' is never required as it can be in a parent node or a
parent node itself can be an interrupt provider. Where exactly it lives is
outside the scope of a binding schema.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Bartosz Golaszewski <brgl@bgdev.pl>
Link: https://lore.kernel.org/r/20220107031905.2406176-1-robh@kernel.org