dt-bindings: spmi: spmi-pmic-arb: make interrupt properties as optional

Make all interrupt related properties as optional instead of
required.  Some boards do not required PMIC IRQ support and it
isn't needed to handle SPMI bus transactions, so specify it as
optional.

Signed-off-by: David Collins <collinsd@codeaurora.org>
Signed-off-by: Fenglin Wu <quic_fenglinw@quicinc.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1651108369-11059-9-git-send-email-quic_fenglinw@quicinc.com