Add rtc reset function.

Add rtc reset function.

On reset:
Periodic Interrupt Enable (PIE) bit is cleared to zero
Alarm Interrupt Enable (AIE) bit is cleared to zero
Update ended Interrupt Flag (UF) bit is cleared to zero
Interrupt Request status Flag (IRQF) bit is cleared to zero
Periodic Interrupt Flag (PF) bit is cleared to zero
Alarm Interrupt Flag (AF) bit is cleared to zero
Square Wave output Enable (SQWE) zero

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>