soc: qcom: smsm: Support using mailbox interface

Add support for using the mbox interface instead of manually writing to
the syscon. With this change the driver will attempt to get the mailbox
first, and if that fails it will fall back to the existing way of using
qcom,ipc-* properties and converting to syscon.

Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Link: https://lore.kernel.org/r/20240606-smsm-mbox-v2-2-8abe6b5f01da@z3ntu.xyz


Signed-off-by: Bjorn Andersson <andersson@kernel.org>