For no apparent reason (as there's just one RPM per SoC), all vregs
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 a slight uptick:

Total: Before=44008, After=44080, chg +0.16%

However the saved n * sizeof(ptr) for every dynamically allocated
regulator quickly makes up for it.
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Link: https://msgid.link/r/20240109-rpm_vreg_cleanup-v3-1-fa0201029f78@linaro.orgSigned-off-by: Mark Brown <broonie@kernel.org>