dt-bindings: arm: msm: kpss-acc: Make the optional reg truly optional

dt-bindings: arm: msm: kpss-acc: Make the optional reg truly optional

The description of reg[1] says that register is optional. Adjust
minItems to make it truly optional.

Fixes: 4260ddfb6496 ("dt-bindings: arm: msm: Convert and split kpss-acc driver Documentation to yaml")
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230627-topic-more_bindings-v1-9-6b4b6cd081e5@linaro.org
Signed-off-by: Bjorn Andersson <andersson@kernel.org>