pm: mark pm_device_runtime_auto_enable as boot function

When demand paging is enabled, only a minimal set of functions
are available at boot. Anything not marked as boot functions
would not be loading in memory at boot which would result in
page faults when jumping to those functions. However, at early
boot, demand paging has not been enabled yet. So we need to
mark necessary functions as boot functions so they are placed
in the correct linker section where they are loaded at boot.

Fixes #56414

Signed-off-by: Daniel Leung <daniel.leung@intel.com>