drivers: pwm: pwm_mchp_xec: Fix u32_t overflow for freq calculation
drivers: pwm: pwm_mchp_xec: Fix u32_t overflow for freq calculation

Fixes u32_t overflow during intermediary calculations using u64_t for
it. on_off is temporary value used for calculating on and off and it
got overflowed with simple test in tests/drivers/pwm/pwm_api
([period]: 2000, [pulse]: 2000)

Signed-off-by: Andrei Emeltchenko <a6565233ddc88e4fb9c66c1d70743223493f2ed4@intel.com>

nashif/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr