power: pmic: add dm style definitions if not CONFIG_IS_ENABLED(POWER_LEGACY)

This avoids an error in converting to CONFIG_IS_ENABLED(DM_PMIC).
Many boards SPL code needs these definitions to compile, even if
the functions are not linked.

Signed-off-by: Troy Kisky <troykiskyboundary@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>