arm: tegra14: bb: use mem_req_soon for emc dvfs

arm: tegra14: bb: use mem_req_soon for emc dvfs

Use mem_req_soon signal instead of mem_req signal to
remove EMC clock request. This prevent EMC drops for
short BB hibernate periods.

Bug 1323192

Change-Id: I6dc2e6a4069e5fcbfad0481860b8a9a691e3c0ab
Signed-off-by: Vinayak Pane <vpane@nvidia.com>
Reviewed-on: http://git-master/r/249419
(cherry picked from commit a23b66a54bc335e426cebf2441ec8382b5b50e06)
Reviewed-on: http://git-master/r/254496
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Steve Lin <stlin@nvidia.com>