mfd: arizona: Refactor arizona_poll_reg

Currently, we specify the timeout in terms of the number of polls but it
is more clear from a user of the functions perspective to specify the
timeout directly in milliseconds, as such update the function to these new
semantics.

Additionally, arizona_poll_reg essentially hard-codes
regmap_read_poll_timeout, update the implementation to use
regmap_read_poll_timeout. We still keep arizona_poll_reg around as
regmap_read_poll_timeout is a macro so rather than expand this for each
caller keep it wrapped in arizona_poll_reg.

Whilst we are doing this make the timeouts a little more generous as
the previous system had a bit more slack as it was done as a delay per
iteration of the loop whereas regmap_read_poll_timeout compares ktime's.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>