spmi: Add driver shutdown support

Add new shutdown() method.  Use it in the standard driver model style.

Link: https://lore.kernel.org/r/1603187810-30481-2-git-send-email-hsin-hsiung.wang@mediatek.com


Signed-off-by: Hsin-Hsiung Wang <hsin-hsiung.wang@mediatek.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20201210023344.2838141-4-sboyd@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>