touchscreen: cyttsp-i2c-qc: Add low power mode functionality

Low power mode changes are made in system information mode,
so modify ISR to check for completion of transition into this
mode.  Then, modify values and transition back into operating
mode.  Enable handshaking so that no communication is missed.

Change-Id: I0692e13810ab4aa88f8626d75a4e7f1ceb885a71
Signed-off-by: Amy Maloche <amaloche@codeaurora.org>