[ARM] 5087/1: Get the PWM layer to handle clock enable/disable properly.

Allow pwm_enable()/pwm_disable() to be called as many times
as the driver wants (and not even count them).
The PWM model is different from things like the clock API
where we need enable counting, because PWMs have one
exclusive user per PWM whereas the clock API can have
multiple users of the same clock.

Acked-by: eric miao <eric.miao@marvell.com>
Signed-off-by: Robert Jarzmik <rjarzmik@free.fr>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>