drivers: cpuidle: lpm-levels: Remove conversion to SCLK cycles

The system_pm driver takes sleep time in microseconds. The programming
in SCLK was required by earlier SOCs that expected sleep time in SCLK
ticks. On newer, chipsets this division results in a shorter sleep
duration being programmed at the System timer.

Remove conversion from microseconds to SCLK cycles.

Change-Id: I8fc0449f051de9650831af49a2315fb158660c0c
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>