RSE: Fix RMA mode boot error

Because the hardware keys are write-only (read as zero) in RMA ICS, the
OTP wiping code was failing with a write validation check failure. Alter
the wiping code to use an unchecked write in order to prevent this.

Change-Id: Ibbd8812a46e79948ead56791e7b12c73008d5d7d
Signed-off-by: Raef Coles <raef.coles@arm.com>