refactor(trng): discarding the used entropy bits

This patch discards all the used entropy bits from the global memory
pool after being delivered to the requester (lower exception levels) by
overwriting them with zeroes.
It effectively implements the requirement, as part of TRNG FW interface
listed at DEN0098 (section 1.2).
https://developer.arm.com/documentation/den0098/latest



Change-Id: I447cbccc1a8ad972418a3569c99f010189d4b2f6
Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>