doc: Correct CPACR.FPEN usage

To avoid trapping from EL0/1, FPEN bits need to be set 0x3, not
clearing.
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Change-Id: Ic34e9aeb876872883c5f040618ed6d50f21dacd0

docs/design/firmware-design.rst

@@ -369,7 +369,7 @@ Architectural initialization
 
 For AArch64, BL2 performs the minimal architectural initialization required
 for subsequent stages of TF-A and normal world software. EL1 and EL0 are given
-access to Floating Point and Advanced SIMD registers by clearing the
+access to Floating Point and Advanced SIMD registers by setting the
 ``CPACR.FPEN`` bits.
 
 For AArch32, the minimal architectural initialization required for subsequent