arm/dt: 8974: Add delay to L2 retention sequence

When L2 SPM executes retention sleep sequence, the L2 SPM could gate the
L2 clock before the Krait SPM completes the retention sleep handshake
with L2. Add a delay to L2 retention sequence to ensure L2 handshakes
with Krait SPM.

Change-Id: I4df17494169ef418e327f116a78e225c2a0781ca
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>