pwm: rcar: Improve accuracy of frequency division setting

From: Ryo Kodama <ryo.kodama.vz@renesas.com>

When period_ns is set to the same value of RCAR_PWM_MAX_CYCLE in
rcar_pwm_get_clock_division(), this function should allow such value
for improving accuracy of frequency division setting.

Signed-off-by: Ryo Kodama <ryo.kodama.vz@renesas.com>
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>