clocksource/drivers/fttmr010: Parametrise shutdown

In preparation for supporting the ast2600 which uses a different method
to clear bits in the control register, use a callback for performing the
shutdown sequence.

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20191107094218.13210-2-joel@jms.id.au