ARM: tegra: enable averaging for P1761 powermon

ARM: tegra: enable averaging for P1761 powermon

Enable 4 sample averaging for P1761 power monitor in triggered
mode. The configuration for triggered mode after this change is as
follows:
 * All channels enabled
 * Conversion time for VBUS and VSHUNT 140us
 * Averaging over 4 samples

Change-Id: I419cf268f2c5995c6b83002b1ec7b933b4080993
Signed-off-by: Timo Alho <talho@nvidia.com>
Reviewed-on: http://git-master/r/393913
(cherry picked from commit 930e8ede6aa15ea17271e4540ad4c3ccba68a72d)
Reviewed-on: http://git-master/r/395108
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Steve Rogers <srogers@nvidia.com>
GVS: Gerrit_Virtual_Submit
Reviewed-by: Juha Tukkinen <jtukkinen@nvidia.com>