Add the mailbox compatible for the IPQ5332 SoC.

Since the IPQ5332 mailbox is compatible with the IPQ6018, lets create the
fallback to ipq6018 compatible, so that we don't bloat the of_device_id
table in the driver.
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Kathiravan T <quic_kathirav@quicinc.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>