rtc: tps6586x: Fix error in RTC tick calculations

rtc: tps6586x: Fix error in RTC tick calculations

In the TPS6586x PMU/PMIC RTC support code, when
converting from seconds to ticks using a shift
operator, the most significant bits were being lost
due to seconds being a 32-bit value and ticks being
a 64-bit value.  A hard cast was added to avoid this
loss.

Reviewed-by: Lowell Dennis <ldennis@nvidia.com>
Tested-by: Lowell Dennis <ldennis@nvidia.com>
Reviewed-by: Jonathan Mayo <jmayo@nvidia.com>
Reviewed-by: Peter Zu <pzu@nvidia.com>
Reviewed-by: Bharat Nihalani <bnihalani@nvidia.com>
Signed-off-by: Robert Morell <rmorell@nvidia.com>
Signed-off-by: Colin Cross <ccross@android.com>