board: 7627a: Update the GDFS residency values to optimal value.

The current residency setting of 6msec for GDFS is way too high to be
entered during active test cases like video, gfx. We profiled this setting
for optimal value on Strider and found 500usec is large enough without
sacrificing the power savings. Hence this change updates the setting to
500usec.

Change-Id: I62bd5431745462e6fdd373cea1d50ccd74b8152d
Signed-off-by: Krishna Vanka <kvanka@codeaurora.org>