regulator: qcom-rpmh: fix build after QCOM_COMMAND_DB is tristate

regulator: qcom-rpmh: fix build after QCOM_COMMAND_DB is tristate

Restrict REGULATOR_QCOM_RPMH to QCOM_COMMAND_DB it the latter is enabled.

Fixes this build error:
  microblaze-linux-ld: drivers/regulator/qcom-rpmh-regulator.o: in function `rpmh_regulator_probe':
  (.text+0x354): undefined reference to `cmd_db_read_addr'

Fixes: 778279f4f5e4 ("soc: qcom: cmd-db: allow loading as a module")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Lina Iyer <ilina@codeaurora.org>
Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Reviewed-by: Lina Iyer <ilina@codeaurora.org>
Link: https://lore.kernel.org/r/20201225185004.20747-1-rdunlap@infradead.org
Signed-off-by: Mark Brown <broonie@kernel.org>