pinctrl: qcom: Hook pm_power_down for shutdown support

Assign pm_power_off() if we have the PS_HOLD functionality so
that we can properly shutdown the SoC. Otherwise, shutdown won't
do anything besides put the CPU into a tight loop. Unfortunately,
we have to use a singleton here because pm_power_off() doesn't
take any arguments. Fortunately there's only one instance of the
pinctrl device on a running system so this isn't a problem.

Cc: Pramod Gurav <pramod.gurav@smartplayin.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>