partitions: dpe: shift context nonce as an unsigned value

Prevent possible undefined behaviour in case of integer promotion.

Suggested-by: Patryk Balicki <patryk.balicki@arm.com>
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Change-Id: I362517815148c53c15045af70a175ef3ddae4935
1 file changed