clk: pwm: Use 64-bit division function

Since the PWM framework is switching struct pwm_args.period's datatype
to u64, prepare for this transition by using div64_u64() to handle a
64-bit divisor.

Also ensure that divide-by-zero (with fixed_rate as denominator) does
not happen with an explicit check with probe failure as a consequence.

Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>