drivers: clock_control: microchip: sckc: optimize get_rate, get_status

Optimize sckc_get_rate() to be called without configurations in
parameter, the selection of slow clock of the timing domain directly
comes from the register.
Optimize sckc_get_status() with return value "CLOCK_CONTROL_STATUS_ON",
slow clock is always on either driven by the RC oscillator or by the
32.768 kHz crystal oscillator.

Signed-off-by: Tony Han <tony.han@microchip.com>