drivers: soc: rpmh: Set up next wakeup correctly

The wakeup value for the system is a tick value based on global timer
for the expected wakeup. Idle drivers provide the sleep interval in
usec. Convert that to a global timer tick and set that with the RPMH
block.

Setup the wakeup value on the hidden TCS with the higher order bytes in
Data0 and low order bytes in Data1. Also simplify writing the data into
the registers.

Change-Id: I2e6dde0382d4eea47e45723f273889c32408baa7
Signed-off-by: Lina Iyer <ilina@codeaurora.org>