For no apparent reason (as there's just one RPM per SoC), all RPMPDs
currently store a copy of a pointer to smd_rpm. Introduce a single,
global one to save up on space in each definition.

bloat-o-meter reports:

Total: Before=92010, After=91062, chg -1.03%
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Link: https://lore.kernel.org/r/20231227-topic-rpmpd_cleanup-v1-1-860ab141b076@linaro.orgSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>