pch_phub: Build context save/restore only for PM

The pch_phub_save_reg_conf() and pch_phub_restore_reg_conf() functions
are only used for suspend/resume support (i.e. when PM is enabled). If
PM is disabled they don't need to be built.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>